>>718089816
>We don't even see any evidence that the Knight can use ice magic.
I mean The Knight is obviously a Holiday (it's Dess) and Carol is so strongly associated with ice that the room chills when she walks in and her hands are cold to the touch. And the part of the TV World where you encounter The Knight is specifically themed around Christmas, snow and cold. It's not much of a stretch.
It's not really the point though, it's just an explanation that's there if you're shonenbrained and buttmad about Undyne jobbing to The Knight. It's not necessary to explain how The Knight overpowered Undyne when The Knight is just physically bigger and stronger than her.