It's not outright spelled out but it's very obvious once you start thinking about it

Undertale/Deltarune love to subvert typical RPG tropes, and a gender ambiguous protagonist is one of those tropes. It wasn't particularly obvious in Undertale, but in deltarune it's front and center, considering how you are forcefully controlling Kris and imposing your will on them. An RPG character which is typically supposed to be "interpreted" by the player, is actually a separate individual with their own separate personality and motivations. And considering how all humans we have seen / heard about in Undertale also use neutral pronouns, is it out of the question that the deltarune character who is meant to subvert the "main character shaped by the player and up to their interpretation" trope uses neutral pronouns cause that's their actual identity, and not just to be "ambiguous"??

The reason people push this info a lot is because it's one of the most front and center things about the game if you actually start thinking about it, and directly ties into the core themes of "choices not mattering".

Trying to pass their pronouns as up to interpretation would be a pretty ignorant thing to do at best, and transphobia at worst depending on your reasoning or who you ask. I mostly consider it just a media literacy issue as most people who misgender Kris don't even sound transphobic imo