From the Wikipedia.Just as an aside, the bit about the Mab being unable to lie, is a shortcut. More properly she can't speak a direct lie. Here is an example of a lie of omission from Summer Knight. It's a play on his expectations.
GS does not have the structure of a lie by omission. Here is an example of a lie by omission: "If you enter this deal, I, Mab, promise I won't hurt you as punishment for rejecting my request for a favor (but I don't promise I won't hurt you just for spite)." Here is an example of a direct lie: "If you enter this deal, I, Mab, promise I won't hurt you as punishment for rejecting my request for a favor (unless I disagree with your reasons for doing so)."
The GS quote takes the form of the latter: "He is mine to shape as I please (unless he exercises free will)."
Also, again, it doesn't really matter how and why Mab was able to say what she said. What matters is that Uriel would not have said she her words were lies when he knows she can't lie and when he knows that Harry knows she can't lie. Here's the form of the seven words: <Factually questionable assertion that has to be justified by equivocation>, <very important truth>.
Here's the form that Uriel would have used: <Factually accurate assertion>, <very important truth>.
Here are examples of how that could have been done using only seven words:
Wrong, . . .
No, . . .
False, . . .
Incorrect, . . .
Exaggeration, . . .
Hyperbole, . . .
Deception, . . .
Deceiver, . . .
The speaker chose the worst possible word to use in the situation. Cosmic master minds of manipulation don't do that.