Some of the local polities were fairly large
They were most definitely smaller on a larger scale of things. Most PNW polities were hardly even big complex chiefdoms in the larger scale, seeing as we have immense ones in the American East.
and the combination of ridiculously abundant salmon fishing and forest gardening could sustain populations as large as anything the Ainu ever developed.
Salmon fishing is not sustainable for larger populations as agriculture is. I do not believe there are any state societies that were not agricultural (and yes, pastoralism is a form of agriculture).
It's never a bad thing to add to your production base/argipackage, but I doubt it will be as decisive as you might think.
No, I think iron+crops will be more decisive in the PNW than New World crops were in Zomia or the New Guinean Highlands. Just think about how productive grain-based agriculture is in terms of sustaining a larger population (I won't deny that agriculture is very rarely beneficial for the individual, but in conflict it's the collective that matters). Just think about the societal changes that sweet potato (more productive than taro) brought about in New Guinea. ETA: this included, for example, emergence of a class system, less warfare, reduced cult activity, increase in intertribal marriage, and a complete economic restructuring in which the generally accepted currency began to shift to pearlshells rather than pigs.
Also, AFAIK the PNWers wouldn't be adding to their agripackage. They would be making one anew, because unlike the Gunditjmara in Aborigine their salmon harvests weren't aquaculturally induced.
And of course, the local nations built some really large boats as well that were capable of raiding up and down the coast quite some distance.
Don't deny this, but PNW activity was much more limited.