When they say "poc existed in those places", they mean that poc were actually a part of those societies (i.e. black Englishmen, etc.) not that they were natives driven out. That's what the user medievalpoc's entire blog is about. Basically, because poc were members of those cultures, white people can't claim them as "white culture". Another user argued that "white people lost the right to having their own culture when they took over the world".