The "problem" with the view that a larger German force can not be supported in Libya is that it fails to take in consideration that the British managed to do so in late 1942 and early 1943 when the 8th Army was there. Using the same ports with the same "insufficient" capacity.
So much for van Creveld.
Just because he writes a lot of books doesn't necessarily mean he knows what he's talking about. As one of my professors used to say "theory is nice but there will always be some emphirical proof screwing up a good theory".
The comparison is inappropriate.
Tripoli could handle 5 cargos and 4 troop ships at a time – when undamaged. By mid-1941, this theoretical capacity was already reduced to 50% (Giorgio Giorgerini, La guerra italiana sul mare). That is because of continuous Allied attacks, which never ceased being a threat to Tripoli, not to mention to other ports. Once the British came in control of Tripoli (and of other ports in late 1942), the Axis was no longer able to significantly damage the ports' handling capacity. After the initial repairs, the Allies were able to re-establish the full capacity.
During the war in the desert, the Axis always employed coastal traffic to supplement the logistical movements from Tripoli to the front. This traffic was never exempt from the danger of Allied attacks, especially from the air and from submarines. By the end of the North African campaign, conversely, the danger from air attacks on Allied shipping, be it coastal or not, was non-existent and the danger from subs negligible.
The British, during their advance into Libya, extended West their railway line from El Alamein. While it did not get all the way to Tripoli, the mere fact that they did that – while the Axis always had to rely on expensive, fuel-burning fleets of trucks – shows the different tack on the issue of logistics, and goes to show how much the situation was different.
Even after taking into consideration all of the above differences, still Montgomery had a very serious logistical problem after the fall of Tripoli, on January 23rd, 1943. He had to temporarily remove all of the motor vehicles from a whole Corps (the X) just in order to keep his force supplied. In other words, at that point, that Corps had the same problems as Graziani's marching infantry in 1940. It took the British a month (until mid-February) to send _one division_ across the Tunisian border.
I think these facts are the empirical proof that the theory opposite to that of Van Creveld is wrong.