Nomads I think usually refer to people who don't farm at fixed land plots and move around to gather food or herds glazing the next patch of pasture.

It was a historical fact that during (early?)qing era, the manchus forbade the hans/chinese to migrate to manchuria,  did those people took a more difficult route to sail to N.Japan?

(they were not the same as S.E coastal chinese who moved to some  S japanese islands)

I don't think the soviets deliberately erased the records of such migration.

The most reasonable explanation is N.E asia had/has a number of  small and dispersed tribes, they might or might not be that cohesive and they might not have pledged legion to any countries. I would not label those ainu as 'chinese'