My Friend Knight theory stands on foundation of the Knight being connected to Gaster or even being his follower
You could even say, that Papyrus Knight implies that Papyrus is a sorta goner and laughs from pain like Friend
But if we believe in Sans is from Deltarune hypothesis, it makes no sense, why would Papyrus transform from a black man to a pale man
Plus, with the theme of Shadow Crystal Bosses "giving themselves to you" by transforming into an Armor/Weapon, it seems as if the Knight itself is made out darkness, just like the Black Knife that it's holding