yeah but a contract also requires a "meeting of the minds" in order to be consider valid, e.g. Mab knew exactly what he meant, his intent and went with it anyway.
Still, we're not talking Magical Law...and Fae are masters at twisting it all to suit them.
But in any event, Mab knows that Harry is his own person and he will not go quietly into the night if she gives him an order like that. Still, for her to tell him to do it, you know there are some possibly dire consequences...
But lets hope by the end of things, Harry can untangle himself and Molly out of Winter's grasp.