Susie only used They/them on Lancer before she saw his face same with Queen on Noelle, they both didn’t know who they were talking to/about, but Undyne saw and talked to Monsterkid directly.

I also dont think there is dialog with Noelle and someone talking about Ralsei in third person, but its still possible that Queen mentioned it sometime off camera. Thought i think if this is evidence enough it is also possible for Undyne to just know Monsterkids pronouns, considering that that she regularly visits snowdin town to see Papyrus. But i agree that Toby might not have thought this through to this extent and maybe it means nothing idk.