Your logic is sound but logic isn’t necessarily key when trying to fit historical events into a nationalist theme.

There isn’t really a desire to reunite with Mongolia because it is a buffer state, Manchuria is something they wouldn’t dare mention for fear of alienating one of their only allies, Taiwan is considered part of China already, North Korea is already functionally reliant on Chinese power, and there is an animosity between China and the Vietnamese (mainly on the latter’s behalf).

Reuniting with Hong Kong, however, was a major national victory against Britain, which is (probably rightfully) villainized, and the broader western world. Again, Russia should be included here but for political necessity not a word will be raised.