Its not like they could pull units from elsewhere. With the recent mainland europe numbers, I don't think they can afford to pull out units from there either.
And its FAR more important to have stable supply in europe and NA because at this point switch already sold half the units the ps4 has LTD. Ergo, its already rooted in the japanese market.
Also if they are stocking up for a chinese realese...
You make a good point about NA and Europe. It wouldn't be wise of them to be stocking up for a Chinese release though when they are still supply constrained elsewhere. There is a chance they are doing this, but I would bet against it.