Not that I want to go full pedant here, but if this is your argument, then you pretty clearly don't properly understand the difference between a "Retroactive Continuity Change" and "Expanding Lore".
The existing lore is that Kalimdor Centaurs came from a specific source: A mating between Zaetar and Theradras.
A retcon to that lore would be something like saying "oops, we were wrong, it was actually Cenarius and Theradras, our bad, change all existing lore to fit the new info."
Expanding the lore by going "hey, have you ever considered that Kalimdor might not be the only place that has Centaurs?" is a COMPLETELY different thing. No Retcon involved, because, at least to the best of my knowledge, it has never been explicitly stated or implied that Centaurs only exist in Kalimdor and nowhere else.