immediate overarching thoughts on Deltarune chapter 2, spoilers obviously:

It seemed really clear to me by the time I approached the end that this Knight character was just Asriel; the unfinished game on Asriel’s computer at the beginning obviously implies that Undertale itself is Asriel’s creation (which follows on from the whole of Undertale just being a “game” Flowey was playing with us), so it would make sense that the game worlds that make up Deltarune would also be Asriel’s creation. But the final scene basically debunks all that… it’s possible that Kris and Asriel are both going around opening fountains, and Kris did just have the mechanics of the fountains explained to them earlier that day… but it seems less likely.

Regardless it does still seem to me like the dark worlds represent Undertale the game. The scene following Queen’s defeat where the kids express how much they prefer the escapist adventure where nobody can tell them what to do to real life struck me as a reflection of the personal relationship people formed with their Undertale save files… the fact that the whole game is sort of an expanded version of Undertale’s epilogue, where all the characters you collected over the course of the game get to hang out in a peaceful utopia-slash-zoo you’ve built for them, only reinforces this hunch.