That's rich coming from the same fanbase who has the same headcanon issue as UT/DR.
In fact, the issue which Deltarune has is a reflection to what Touhou was post-PCB.
Since in that era, almost everybody with their dog here always portrayed the fan interpretation of each of every character as if it was fact, instead of just doing it for the sake of fun. Because I know that some people didn't played or at least watch a walkthrough of the games (which funnily enough, not even the creator seems to care if the retro games are any indication, plus it's more accessible now thanks to fans, I mean where are we right now?). It's one thing if it was open-ended and left to interpretation, except that we already got a grasp of what the characters are, by that it's more insulting to claim something which they are really not. This kind of mentality didn't just taints new blood who got in for the first time, but also gaslights long time fans into thinking that THIS is what they're actually like.
Cirno and Alice are the most notorious example and the poster children of what am I talking about. The former got reduced to a joke by the crimes of being just energetic and silly. While the latter got reduced to the most degenerate shit I've ever seen, all for the sake of a ship which didn't exists in canon.
Of course, even back in the day, the fanbase wasn't as horrid as Undertale, but I wouldn't call it "perfect" either.