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'