There's lots of criticism I'd have for Chinese policy, but this terminology isn't one of them. It's very common to use the term "minority" to refer to a group that is a minority at the national level, but could still be a majority in some local level.
This is true in the American context as well (Native Americans, Native Hawaiians, Mexicans, etc.)
If someone believes Tibet to still be an occupied country rather than a part of China, it wouldn't be accurate to call Tibetans a minority.
So's Hawaii, but you don't see anyone giving a damn about that.
The Native Americans were sovereign countries for hundreds of years until they were invaded. Their land is currently occupied against their will. If someone believes Native American land to still be an occupied country rather than a part of United States it wouldn't be accurate to call native Americans a minority.
The timing matches too: the Chinese ended Tibetan independence in around 1720, around the same time European settlers conquered Native Americans.
e.g. apartheid is often referred to as "white minority rule" - that is the minority oppressing the majority.
Similarly, this gets a little subjective. It's not clear to me in modern liberal democracies that women are a minority politically, because they do in fact have the ability to set policy equally to men (even if they happen to be less present in the political ranks)
No. If a country annexes a smaller nation, they don’t get to call the invaded population ‘a minority’ just because a stroke of their totalitarian pen made the countries one.
It is true in the American contexts you cite, but you are there you are talking about the same land - not additional grabs.