Man the pope is a moron, his whole plan was drop the princess into a hole and just leave, hoping she would starve to death.
And not once did he think, maybe she came here because of that place or that she could have told somebody she was going down in the basement, and didn't even stop to think that maybe she would be able to crawl out of the hole again with her magic.
Even if his plan is to expose her as the witch if she gets out, he has no way to prove the way she got out was not done without witch powers.