you know, it always struck me as odd that noelle told kris about them visiting her house at night and removing the thorn ring, even out of the context of them explicitly telling her not to tell them about that. i was racking my brains about it, wondering if she was telling them to affirm to herself that that actually happened and it wasn't a dream -- maybe hoping they'd confirm it for her -- or out of sheer curiosity in regards to why they even asked her not to say anything in the first place.
but i feel like the answer might be simpler than that: she's used to telling them about things that they've forgotten they did in the past.
having DID causes gaps in memory, which it's already been established kris has (see the egg rooms, and those times where kris does something the player is unaware of when kris removes their soul could also be metaphorically referencing memory gaps). if kris has done something with noelle, and she's brought it up to them afterwards to a blank expression on kris's face, it makes sense that she -- with some confusion i imagine -- would give remind them of the details. and if this happened frequently, it would be something she'd get accustomed to. we've seen her often reflecting on old memories she's shared with kris, asriel and dess, but maybe she's not just doing it for nostalgia purposes -- maybe she's also checking to see if kris remembers that as well.
she may not necessarily know they have DID -- she may just see it as a kris thing, another thing that makes kris themself. yeah, they've got poor memory at times, just one of those things about kris.
it's a loose theory, but that flashback was always something that bothered me a bit because i wasn't sure what purpose it served diagetically (obviously for the player's purpose it was exposition) and it made more sense when i looked at it this way.