He was only referring to weapons made in the Imperium, weapons that archaeological findings indicate to have been made by Rome.
There's a problem there. What was found at Ejsbol was around sixty swords, for roughly two hundred spears, shields and javelins.
That being deposited at different times.
Either the argument is that all or essentially of these weapons were Roman-made, either we're talking about a part of the find, which I suppose (maybe wrongly) is essentially composed by swords (in relatively limited numbers) : I tried to find a detailed file about archeological data from this cache, to no avail unfortunatly.
Can't see how would someone would do such a scheme without being caught
By the simple fact that the Imperium had a hard time producing enough weapons for themselves, also such illegal commerce would depend on too many factors to be in large numbers.
As you said, imperial control was limited, and far from being omniscient.
Even outside periods,like the IIIrd century, were imperial control was more of a private joke than a reality (that Diocletian had to reaffirm the imperial control over state manufacturies tells about the situation), stealing, corrupting, etc. and smuggle your way up to Barbarians was a fair possibility. Forbidden trade usually means a profitable trade (especially exchanged for slaves or amber).
Don't forget that army weaponry also sub-contracted a lot of its work, which make it even easier to divert weaponry or to illegally produce some.
Don't get me wrong : a lot of trafficers must have been caught and punished, but I don't think you had nearly enough scrutiny to make roman weapon production and distribution a state monopoly.
I went a bit into the Roman legislation (often repeted, which probably means it wasn't that well respected) on selling weapons (or rather, looked a bit at what Jean Andrau said about it, being specialized on roman trade) tough, and apparently it was tweaked in the way that it was forbidden to sell wheat, salt and iron (including weapons) to Rome's ennemies.
It seems that it didn't as much forbidden trading weapons, than trying to speculate on Rome's war at the expense of the empire.
Giving that trade of wheat is really attested, I don't see why weapons trade wouldn't be a main explanation there as well.
Most of the weapons and armour would be damaged by battle and the findings imply that the gear was in good conditions and without remends
These cache weapons are part of a sacrificial ritual, probably directly produced for this intent
Also most Barbarians that acted as auxiliary would be settled inside the Imperium so those weapons would find a hard time getting into Germania.
I disagree : while you had a lot of auxiliaries coming from laeti communities by the IInd, Barbarian auxiliaries were as well recruited (trough treaties, for exemple) outside Romania proper. For example, Iazyges of the Pannonian Plain were bound by treaty to provide 8 000 cavalrymen, and similar treaties were passed with Germania's peoples during the Principate.
Denmark was but an example but if you read about the Himlingøje dynasty it indicates that Rome supported the rise of power in the Baltic, probably to force the tribes that bordered Rome to be more cautious as they now had a power in their backs.
There's a lot of room between the rise of a local complex chiefdom in Zealand, supported by Romans always in need of clearly identified and identifiable interlocutors; and a strategic plan to "rise" a chiefdom into a local hegemôn.
It doesn't seem, for instance, that this Danish complex chiefdom as a putative ally was hugely useful in the IIIrd when it comes to a rear alliance, when it was undergoing its apogee.
Also I never said diplomacy of destabilization, I said that they supported the raise of specific tribes that they considered loyal to help them in keeping the border tribes under control, it would only be in times of war that they would expect their "allies" in Germania to attack.
Fair enough, but you'll admit how you formulated it* was a bit misleading.
Still, I don't think that such planned policy could really account for the presence of Roman weapons (Roman-made or copies) as a coherent policy for the already mentioned reasons, as well that weapon giving covered as well "propagandist" use (Roman state and Imperial figure as source of wealth was a regular theme of imperial culture), especially giving that the weapons we mentioned so far were discovered in sacrificial caches, and then less than probably used as warfare.
Does that means that Romans were unaware they allowed barbaria kingships to recieve the means necessary to their stability (the scacerdotal of Germanic kingship being agreed upon, I think)? Maybe not.
The point being that you had a lot of other reasons, especially for "remote" peoples as in the Vistule's basin, to have access to Roman weapons without something entierly planned for centuries.
*they armed them to encourage fights between the Germanics.
And you can see the Roman policy in the IIIrd century. During Caracala and Alexander they kept the border in peace so Rome had to interest in supporting inter-Germanic war
Which is contradictory : a peaceful limes was as well based on Romania being spared inner crisis, than Barbaricum being less troubled by inner conflict (as pointed by the Marcommanic Wars).
Not that rear alliances are unthinkable, at the contrary : the conquest of Dacia points to an extensive use of Sarmatian peoples by Trajan (which created another issue, as Sarmatians raided the newly conquered regions shortly afterwards). But I'm quite under the impression it's really a thing when Rome intervenes directly in the Barbaricum.
Another interesting example of Goldsworthy is that he says that the Roman control over their "allies" was very tenue and that they had trouble ensuring that they kept the peace when Rome wanted, and that they were forced to bribe them to stop attacking the border tribes, which is a bit ironic considering that they had bribed them in the first place
In short, they had little control beyond the periphery (and even there...), and whatever they could pull off had little incidence safe structurating Barbarian chiefdoms and complex chiefdoms.
At this point, either Romans were particularily thick, or they didn't as much planned alliances than trying to find legit interlocutors (not only in the eyes of Germanic society but as well, and probably more importantly, to their own).
Does it go against the evidence of the policy of subsides of Rome? I agree it doesn't. The point, there again, is the presence of clear policy of rear alliances trough giving weapons away.
I was at a interest convention/reunion of several former Portuguese and Spanish army officers and military historians and one of the speakers spoke of this and indicated that the Romans only considered short term gains and that their arming of the tribes behind the border was one of the things that came back to bite them in the ass.
One of the main problems that Romans had, IMO, when it come to Barbaricum is that they had only a faint idea what really existed there. Tacitus' Germania remained the basic ethnographic work for what Romans could be bothered with, and it guided their "Germanic" policy a bit too long, conceptualizing Barbarians as deprived of history (something that only Romans, Greeks, and maaaaaaybe Persians and Egyptians, in a way, had) and therefore of genuine social complexity evolving trough time.
Now, how did it translated in terms of "geopolitics"...that's more or less the (interesting) discussion we have.
I'm a bit wary about the focus on short term gain : it's true in time of crisis when they had to make choices (not all good, far from it, and indeed deprived of strategical background), but the relative success on some fronts makes me think you had to have long-term goals, at least along the Rhine and in the African limes.
On the Glossary he provides this source: W.Willems, Romans and Barbarians (1986).
I don't have it, but I'll try to gave it a look.