Yeah, I read that , (in my ladybird book on Joan).
Two possibilities.
1) She had divine contact with Yahweh and he told her.And its true.
2) The Nobility and clergy of the time did not beleive her, but they needed some quasi-easy to say- impossible to disprove miracle, and simply tagged it on. They were working on a french prophecy that a "maiden" would save them from teh english, Joan fitted nicely.