It's a possibility, at least. Ultimately, it could be either reason because they both make sense. I mean...when you step out of a shower, you're colder than you would be if you were dry, right?

And if these little guys are constantly slimy, I can see why they might not be crazy about extreme temperatures in either direction. Too much heat means they dry out, not enough means they freeze.

[It...makes sense to him, at least.]

But, on the other hand, typing can obviously account for the same thing when it comes to Pokémon.