Big shit:
-Post-genocide "next world" theory debunked
-Deltarune's characters are completely separate from Undertale's (this may debunk the prequel theory as well, since that theory banks on the fact that Deltarune Sans and Undertale Sans are one in the same. He's either an exception, or he's just a lame, possibly non-timecop Sans, we'll see)
-Your choices actually do not matter; there will be no multiple endings. Kind of a letdown seeing as one of Undertale's coolest features is how detailed the endings were. Like there were 3 main ending, but lots of little sub-ending of those 3. Like he says, most games only have one ending, but in the context of Undertale, it is kind of a letdown. You might as well not even "ACT" at that point, since straight up fighting is often easier.
Still really looking forward to the rest of the game. There's still a lot of unknown shit, like why Toby was even compelled to make this odd alternate-universe game parallel to making Undertale. I find it kinda strange that, given the fact that according to him, these two universes are completely separate and don't affect each other, he decided to use the same characters we know. I get that that's kinda the point of an alternate universe, but given that Deltarune is a different game entirely with a completely different premise, he could have just made new characters for a new franchise. I think that fact adds some suspicion to something going on between these two universes. It sounds more likely than him just being lazy and wanting to work with the personalities he already knew best. Either way we'll know eventually.