I think instead of looking at vehicle allocation by population perhaps look at allocation between the countries based on past Sienna years.But it's the sales guy who told you 4-6 months, not Toyota. From what I hear, Toyota gives dealerships expected allocations at the beginning of each model year for their planning, so it's not like they're completely in the dark. Sure, things change because of production/supply chain issues, etc. And there's no details on each trim/color, so there's unknowns there.
I can say with almost certainty that the only ones playing games are the dealerships, playing favorites and manipulation customers to assign allocations to whoever is most beneficial to them. Toyota just produces as many as fast as they can. If there is any blame on Toyota, it's that they allow dealerships that power, instead of pulling ordering back to a central system. But that's every manufacturer (except Tesla).
There's another point I looked into. I was a bit annoyed at the whole US vs Canada wait times, and it seemed like Toyota should just allocate more cars to Canada to balance things out. However, looking at the monthly Sienna sales numbers where available, Canada numbers aren't too far off from 1/10th of US. It seems that Canada is just getting its "fair" share of vehicles from Toyota, and our wait times are just our market situation here.
In Canada, Sienna is pretty much the leader while in US it is not and hence more over allocation vs prior years.
Just my 2.5 cents [inflation].