It really depends on what your definition of "lying" is. The fey can stretch the truth quite extensively.
And what Uriels definition of lying is. I assume that because he is speaking to Harry his is a mor human definition.
From Dead Beat:
"You know, wizard, that I may speak no word
that is untrue. Thus is my word given to you."
You can deceive someone by telling incomplete truths leading to a wrong conclusion. Uriel and most humans would consider this lying because while all statements are true the underlying message is not. Language is communication and at the end it is the message that counts.
You can use ambivalent statements or statements that suggest more than they strictly mean. Human communication is full of that and it can make life interesting. But not in this setting. Again Uriel and most humans would consider the underlying suggestion a lie. Like the unseely accords the spirit of the text does not count for the Sidhe. But they know what it does to humans.
If we look at the lie/truth:
“I have his oath, ancient one. What he has given is mine by right, and you may not gainsay it. He is mine to shape as I please.”
None of these statements are untrue. Only the last statement is open for different interpretations. Harrys interpretation is the one Mab wants Harry to believe but it is certainly not the correct one.
Like a sculptor and a piece of stone you are restricted in shaping it by the nature of the stone. The ways you can do so are also restricted by practical considerations. Some changes imply other changes you do not want. But nobody is going to interfere Mab trying.
There must be limits otherwise she could take any human and make it into what she wants. She would not have bothered with Harry.
Also while Mab can not change Harrys soul (Which is Uriels prime concern but maybe not Mabs. She just wants him to do what she wants and wants him to want to do so.) Harry can do so and Mab can try to make Harry do so. Harry should have been warned about that. It is what Eb said in Changes:
“She might lean on you pretty hard. Try to put you into a box you don’t want to be in. But don’t let her. She can’t take away your will. Even if she can make it seem that way.” He sighed again, but there was bedrock in his voice. “That’s the one thing all these dark beings and powers can’t do. Take away your ability to choose. They can kill you. They can make you do things—but they can’t make you choose to do ’em. They almost always try to lie to you about that. Don’t fall for it.”
“I won’t,” I said. I looked up at him and said, “Thank you, Grandfather.”
That is what she was trying to do. Eb's warning was not enough but Uriels came at the right moment.
The lie is in the suggestion which does not count as an untruth because That was not Mab's saying. It was Harry himself.
