While we do get that text, as I had pointed out earlier, we do see evidence, in game, that not all of the Zora had changed into the Rito. We can translate menu items, in WW, that tell us there are still Zora present. And, there are items that can be found in later games of that timeline that still indicate that they are around. So, we know from in game information, that not all Zora were changed. Plus, a population of Zora becoming Rito, and a Rito that is quite different from what we see in BotW & TotK, does not preclude the existence of a Rito people before that.