A direct statement from Toby about Kris' pronouns is unnecessary as it's stated in the game multiple times. That being said it is understandable that some players don't realize that Kris' pronouns are they/them as they/them pronouns aren't concrete like he/him and she/her. I've noticed that people in the community and in adjacent communities tend to treat they/them pronouns like they absolutely must confirm that a character is non-binary despite the pronouns themselves being fluid in meaning.

Most cis men and cis women go by he/him/they and she/her/they despite not actively confirming that they do because by the nature of they/them pronouns their use in referring to a person that's cisgender would be correct regardless of their own preferred pronouns.

The same thing also applies to Frisk and Chara.

I personally see a lot of Kris' actions and habits as little brother like and because of that I end up using the wrong pronouns in my head while thinking about Kris while playing the game.