She was lying.
Joan was a naturally skilled tactician but sadly sexist Catholics like Jacob would never give a peasant woman a position of command no matter how much she deserved it, unless she faked getting messages from God. She figured out who the real king was on her own through reasoning (maybe his disguise wasn't all the great) or even got tipped off in advance for the purpose of political propaganda. The king played along because she had proven herself at this point and he didn't want to let her talents go to waste.
Okay I don't know enough about the history to say if that's what happened, but it seems at least as plausible as the idea that the creator of a universe of countless trillions of stars and who knows how many sentient lifeforms gave a giant shit who was chief monkey of one small part of one planet, and the best way she could think of to help the situation was give visions to a peasant girl instead of just striking down the false king with lightning bolts or something.