Well, you kind of actually can take the king, you used the freeze spell/potion in this case, and he can’t move it. Also another way to take the king is if they block your check, you can charm the blocking piece with the jump spell/potion and take the king.
Is this position a forced mate?

So what you're saying is that my opponent could've used a freeze spell themselves and avoid having their king taken?
EDIT:
just had the chance to confirm taking the king is a thing in this variant
https://www.chess.com/variants/spell-chess/game/53906139
EDIT2:
Oh, I just realised. The whole reason I made this topic is because I thought you couldn't analyse in this variant. Turns out I had to go a "move back" from "opponent resigned" and now I could move pieces and test theories. So you can close the topic now
https://www.chess.com/variants/custom/game/53901815/15/3
My opponent resigned in this position, but I'm not sure why. In this variant, you can't "take" the king. So I think in the next move he'd just have to move the king, no?