We are not talking about self-applied labels, since parent explicitly says "are referred to as".

So I think it is a very valid point to wonder why some regions came to be referred to (in american english) as latin america, where others that would in principle fit the logical criteria didn't.

I agree it could be put in a more constructive way, but yourself assuming they are not familiar with a particular culture just because you don't like their comment is what actually comes across as rude and dismissive to me.