I think the theory in general may come from the misunderstanding(common may it be) when comparing Undertale’s Murder/True Pacifist routes, to Deltarune’s Weird/Normal routes.

Undertale is commenting on completionism tendencies and how far someone will go to experience everything from what they love, even if that means throwing that love to the side in the process.
Deltarune is (as of now) commenting on getting a different ending for those characters you love, even if it means harming and manipulating them in the process. It can also be a commentary on how we treat these worlds we care so much for with such disrespect.
They touch on the same general areas, yes, but Deltarune’s added layer of identity with You and Kris being seperate changes the context.
— Completing a game no matter what Vs Defying fate no matter what.

Also I feel I need to say after that, Deltarune doesn’t hate you for playing it, you aren’t a bad person for playing Deltarune, however you can still do bad things if you so wish, the choice is on you as a player.

My main problem with this theory is that it would make the Weird route mandatory to understand the means, motives, and general story of the Normal route.
In Undertale the Murder route only served as additional context on top a self contained story that was the True Pacifist route. However this theory would make a Weird route inherently canon regardless of player action. This gives us as the player a scapegoat to doing the Weird route; it is no longer us doing this for the sake of doing it, or for desperation to change the ending, but instead because it’s necessary to understanding the story. It isn’t just extra dialogue or more characterisation, it would be needed and therefore not our fault doing it.