In the setting of Undertale, humans are in the antagonistic role, having trapped monsters underground after a destructive war. Monsters are defined by their compassion and are explicitly said to be ...

Undertale ’s Human Nendoroid will start shipping in Japan in August 2023. It will ship in the US in Q4 2023. Pre-orders will be open until May 11, 2023.

But if it's true that he has some knowledge of alternate universes, then it's possible the Sans in Deltarune is aware of the Sans in Undertale. Deltarune is out now for PC, PS4, and Switch.