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The Undertale Multiverse is a confusing place to the outsider of the Fandom. Everything is canon to each other due to the meta narrative of how the Multiverse actually works, yet some how multiversal crisis have no effect on the fanstories storied in AO3. This was because the Creators were not just creating, AUs and OCs; they're actually apart of the their greatest creation, Tales.

Tales are collection of ATs, AUs, OCs, and locations whose fate were dictated by the Creators. Although all AUs and ATs are canon to the multiverse by default, their relevance depends on the Creator of said Tales. For example, while Glitchtale exist in Underverse, but is not relevant to its Tale and vice versa.

It’s generally impossible for Tales to intersect without the collaboration between Creators and their authenticity toward each other plots. This means that the events of Interlopers have no effect of Endertale and other AUs that were involved in the Tale itself. In actually, the Creator simply created an clone of said AUs so that it won’t conflict with the Tale that were currently being told by another. This gave them creative liberties on the said clone as they can do whatever they want with it. This also involved interpretations of Undertale/Deltarune OCs as well (as long as they’re permitted by the Creator that created them in the first place).

Although certain Out!codes does have the ability to manifest in any place in the multiverse, they’re unable to influence the Tales outside of their own. For example, CORE!Frisk can manifest in any AU they want, but doing so will simply create a copy of said AU for their own Tale. They can, however, Monitor said AUs; but their knowledge of infinite Tales were limited by the Creator themselves.