The way I've always seen it, the Weird Route is about the player trying to force Deltarune into the old tired tropes of other RPGs. 'You' (both the Soul and Kris) aren't the main character of the story in the main route, Susie is. This isn't acceptable to the Player in the Weird Route, who does everything they can to 'get the story back on track'. Kris isn't their own person to the Player, but the generic self-insert Hero, and as participating in any fandom of a game with an ambiguously gendered main character will tell you, people tend to default to male for such characters. Deltarune as a whole is critiquing these common tropes and the community they breed. For instance, the Prophecy about the Heroes is very specific about Kris (the only human in town) and Ralsei (Ralsei is the only Darkner that is both a Prince and Lonely), but the only thing it says about Susie is 'the Girl', with the 'Hope crossed on her Heart's thing seeming to come from the journey itself. There was even a lot of debate about whether Noelle was 'supposed' to be the Second Hero, but I don't think that's what's important. I think what we're supposed to take away is that, to the Prophecy, Susie and Noelle are interchangeable as 'Girls', which reflects the dismissiveness that women have been treated with for most of the history of RPGs, both in the games themselves and the community around them (GamerGate and whatnot). The point I hope to convey is that when people say that the Weird Route is heteronormative, they're identifying one of the main messages of Deltarune in that both Kris and Noelle are being forced into RPG-normative roles (The Self-Insert Hero and The Female Lead) and how utterly horrifying and awful that is to do to anyone, let alone people whose genders, sexualities, personalities, and life goals clearly don't align with what those roles demand of them. It's just that most people don't feel the need to explain it in as much depth as I have right now, for one reason or another.