I'm going to breeze over everything else, because I think this point has been argued to death on both sides and no further arguments are going to change anything but upset people. However, I want to address @Vyxsis 's closing question:
"why take the position that we must designate one thing "real" and another "not-real" in a world that's all made up? it seems mean-spirited to me, to put it mildly."
There is nothing mean-spirited about players who want to play a fantasy game wanting there to be rules and guidelines to their fantasy game, because in a pretend world, having guidelines of what "is" and "isn't" is essential for players to feel more immersed within it. To make the pretend world feel more real, you need more rules explaining how that pretend world works.
Guidelines aren't directed at anyone or intended to hurt anyone or wound anyone. This specific RP/Lore guideline is the way it has been supposed to be, but because of the dearth of information available to players we have players now feeling like their characters' histories and backstories are being attacked or now made to look illegitimate and that is very unfortunate because for plenty of those players - they didn't have any way to know any better and it isn't their fault. As was mentioned, there is even confusingly mildly contradictory stuff in those HELP ORIGINS files - the parent folder is labeled 'mythos', but it isn't really abundantly clear that this is mythos TO the characters, not "the mythos of aetolia" (when you throw the word 'mythology into a fantasy world, it gets confusing).
However, at the end of the day, in a role-play-heavy and immersion-heavy game that also relies on those aspects having mechanical effects: you need guidelines and this is the one that has been chosen. It is both the simplest to hold up mechanically while giving RP freedom (doesn't stop you from boinking whoever you want and having a happy/dramatic/tragic family). It isn't intended to be mean-spirited or diminish your RP or anyone else's. The line just needed to be drawn somewhere and this is where it is for better or worse.
The thing is, that has been the official stance for a long, long time,* it was just never made abundantly clear like it should have been.
I feel bad for the players who have played the half-breed/more in the past, though, because the lack of access to this information has lead to the problem we have now of people who didn't know and now feel like their RP has been dumped on when it wasn't their fault that no one just made a help file explaining this for players who would have happily stuck with lore cannon if they'd had clear guidelines to follow.
edited for clarity and to add: *This is my understanding after talking to a couple admin on the topic - admin please correct me if wrong.