I had that thought, myself, but since it would in this way disadvantage the person putting the other on ignore it might actually discourage an over-use of ignore that I occasionally see now (being used to get the last word in IC arguments, for example).
Since the idea of ignore is "I have tried to reason with this person and it just isn't working, so I want to have nothing to do with this person ICly or OOCly", letting the person ignoring put themselves at a mechanical disadvantage for sake of truly not having to see the other around their happy-fun-time-entertainment-box activity seems reasonable it to me.
What you do when that happens is politely OOCly point out that tells are IC unless noted otherwise and apologize that you'll have to assume that the conversation had, indeed, been IC and then react accordingly IC about how their character is apparently insane. The more insistently IC you are, the more they'll follow suit.
Ahhh, okay, that's fair! And -even in your further defense- I asked my husband what he would have thought the "plain number one, no cheese" meant and he said 'bun and meat', so yet another point for "Micky D's, you suck'."
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.