This is not Re_Birthday. This is not "Soul of Adam". This is not old plot points that mothy changed his mind about years later (he's been working on this series for over a decade, remember). This is not mothy forgetting about stuff. This is not vague, with us having to read into a bunch of disconnected facts to make it work. This is clear and direct evidence coming from his most recent release.
SCaP featured, at the end, Allen failing to save the world from Punishment. He wakes up where he started, in the box, with no clue of how he got there. The Heavenly Novel mentions that Allen does not do the events described in SCaP, confirming that for some reason, they do not happen in this timeline. Yet, he goes straight for Nemesis, demonstrating that he has knowledge of what happened in SCaP, because he knows she's trying to destroy the world and he goes to stop her. Lychgate then says, "Sickle merged an Allen from a timeline that failed with our Allen, giving him the memories of that failed timeline so that he could avoid the same fate". What else could that possibly be referring to?
I mean, Octo put it kind of well up there (you didn't even really respond to anything she said at all, actually). It makes less sense assuming SCaP isn't the failed timeline than it does if we assume it is.
We have a situation that didn't make sense, and mothy provided an explanation. It seems foolish to not accept the explanation on the grounds that he may, possibly, at some point in the future, say "surprise! actually it was something else entirely!". Especially because we really don't gain or lose anything by doing it. So what if it turns out we're actually wrong about this? All that'll equate to is a little bit of inconvenience on changing the page information back. Inconvenience that people like Octofan are more than happy to deal with.