Well, he did instantly hit the floor, didn't he? Winter law seems to be some kind of natural law. There is nobody really there actively reminding you. Things just fall down without being reminded to do so. I don't believe Mab was actively involved reminding Harry of her power. Cause: The law itself was deactivated by Harry's remark (screw Winter). Effect: Mantel dissolved, Harry hits the floor.
Except that Winter
is Mab. His mantle is Mab's power within him. As Maeve put it to Harry on the bridge, the power within him is Mab herself, which is why she is weak to him. He wasn't just saying "Screw Winter" he was saying "Screw Mab" because they are one in the same. By denying the rules that he was bound to through oath and fealty, (whether he understood this or not), his power through her would be denied him, and I don't see any reason he couldn't be reminded of this via a temporary paralysis reminder. Besides, as I said before, we have direct evidence that Winter Fae can, and have healed physical injuries on a mortal before, specifically Harry. So it stands to reason that a Fae with even more power than the one who healed him in the past, could heal him as part of a bargain. Harry in fact goes into great detail about how hard it is to heal someone via magic, and that it meant that Leah had a direct line of influence/power over him to be able to pull it off. By swearing fealty to Mab as her Knight, and boinking her on the table...well....that's about as "direct line of influence" as you can get