Yeah, if undertale released in chapters it would probably result in similarly frustrating arguments about theories like in deltarune.
Heck, even them being aware of Asriel existing is technically accurate bc the room you sleep in in the ruins is called Asriel’s Room in the code so people would definitely be arguing about that even though Asriel isn’t brought up in-game until New Home. People would definitely know he exists but they’d probably have much less to go off of. They’d argue about his role and how significant he even really is.
Another thing i could see people arguing about if Undertale only had like 2 chapters out and Chapter 2 ended right between Snowdin and Waterfall is the whole thing with the game never really making it explicit that “Asgore” and “Mr. Dreemurr” are the same person at this point (Asgore’s full name is only really revealed during Undyne’s speech) especially since Toriel and Papyrus describe them in two absolutely different ways. There’d probably even be a camp of people who think that “The queen” mentioned by the shopkeeper, “King Dreemurr”, “Asgore”, and “Toriel” are four entirely separate characters. There might be one or two theories theorizing that Flowey is somehow connected with the king and queen due to his “Ill be the prince of this world’s future.” line, but who knows.
Gaster arguments would (possibly?) still happen because the Wrong Number song and the Sound Test fun events are both located during the Snowdin section of Undertale, although I think fun events in general were added later on in the games development so maybe not. And even if they were there the arguments would probably be less about his role or relevance and more about “who even is this guy” since the Gaster followers wouldn’t really even be there to provide context.
I think there’d also be a lot of speculation about Undyne just in general and what kind of character she is. (Although maybe not, considering how little speculation we ever got about Queen before Chapter 2 released, everyone would probably come to a consensus on what they think Undyne is like and settle on that.) There’s probably be speculation on Asgore’s motives for wanting to kill humans as well.
But yeah a significant amount of theories would definitely be way off the mark for a lot of reasons that a lot of deltarune theories are probably way off the mark.
(But even then there was that one person who managed to predict the nature of Toriel and Asgore’s relation to each-other based solely on the name of Asriel’s room in the 2013 Undertale demo, which is impressive.)