The problem is that as Russian Far East demonstrates as soon as a place is filled with white people it stops being Asia.
For example what exactly makes Siberia not Asian but Manchuria Asia?
Why is Indonesia part of Asia including West Papua but Australia isn't?
I am willing to bet that if the Russian Far East was Northern China it would count as part of Asia. Similarly if Russia had ever acquired Hokkaido and filled it with Europeans it wouldn't be "Asian" any more.
Asia and Europe can't be defined geographically instead Europe is where Europeans are, Asia is where Asians are.
Exactly. It would be plausible to have shifted the border between China and Russia around sometime during the 19th Century to result in Russia colonizing a stretch of, say, what is northern Xinjiang or Heilongjiang in OTL and filling it with Russian and Ukrainian settlers. However, from the perspective of that timeline, those same regions that are firmly Chinese (and thus East Asian) to us would then be seen as "North Asian" by virtue of Russian sovereignty.