It was not the Manchus who reconquered Tibet in the 1950s, after it had been an independent country for several decades.

And the general argument is not about whether there is something inherently imperialistc in the Han -- it is about whether the Han are so isolationist that this should somehow prevent China as a political entity from expanding. Well it has not prevented this before (cf. also the Tang period expansion, if we want to talk about more distant history), so I see no reason why it should prevent it now. Unless, say, the CCP cedes control to an openly Han-nationalist party, but then the last one was imperialist alright (https://en.wikipedia.org/wiki/White_Terror_(Taiwan)).