Tibet was de facto independent for a while, but it re-joined China voluntarily circa 1950. About 10 years later, with some help from western agitation and assets, the theocratic ruling class felt too threatened by development empowering their serf population and sought to secede in order to maintain their fiefdom. Mao sent in the PLA and crushed the secessionist revolt.

You really can't "no u" this one because of Imperial China, the PRC's claim to Tibet is completely valid. You'd probably have more luck trying with Xinjiang, though evidently that is viewed as slightly played-out now.

In fact there are NO countries in the world immune to the “you displaced a previous civilization” argument. They’ve all done it.

Yet, some did it much more recently than others, and many of the peoples they stole from still exist and are still being exploited (not China-Tibet case). At the very least, one would expect countries not to go around questioning the legitimacy of other countries' territories when their own have no legitimacy at all.