"I can't promise that entirely. If you do something silly of your own volition, then of course something will happen. But I can at least promise you that I won't let you get stuck."

It's said with total seriousness. Strange might not yet know what sort of magic he could use to get Tamaki unstuck if that were to happen, but he should be able to do something. After all, he knows from experience that being stuck in a tree is amazingly unpleasant. Tamaki's a good kid. Let's not have him stuck and/or traumatized while testing the limits of his magic.

Nowhere in Strange's mind does the option of 'don't do the thing in the first place' show up.