In addition, take into account the comment to which Uriel (because it is Uriel; he's not about to go and rope some lower being into lying for him, especially considering the circumstance with Collin Murphy) responds to:
I'm not suggesting he did. I'm suggesting he used a proxy and didn't have control over the precise seven words used, only that the speaker was limited to seven words and that those words should be used to deliver the message that Mab was incorrect. This would be similar to the TWG inspiring the Bible but not writing it. It's also similar to how Mab used proxies in the very same story. Unlike Demonreach, who appears to have literally spoke through his proxy, Mab merely gave her proxies (Leah and Inez) instructions/inspiration regarding what she wanted Harry to know. Mab's proxies actual words were their own.
Cozarkain - I get the conundrum you're talking about, but I think you might not be giving the idea that Mab's lie is a lie of omission a fair enough shake. Faeries cannot speak lies; however, they do nothing but lie through omission.
In the Mab quote you provided, the first sentence is true, the second is true, the third is false but Mab believes it to be true. The WOJ in the post above yours proves that Mab believes the third sentence to be true and is not merely lying by omission. Also, the question here isn't actually whether Mab was lying. She wasn't, because she can't. The issue is that the speaker called Mab a liar, which conflicts with our understanding that the Fae can't lie.
The first question is whether Uriel could have called Mab's words "Lies" without bearing false witness. In response, others have argued that the word lie has multiple definitions. I disagree that the other definitions apply. The DV, at least when talking about the Fae, uses strictly the traditional definition (lies of omission, by definition in the DV, are not lies). It is fact that the Fae cannot lie, period. The only definition applicable in the DV (at least when discussing the Fae) is the traditional definition. Switching to another definition is equivocating (an informal logical fallacy).
I simply do not believe Uriel, a being with cosmic truth with a perfect understanding of the lie-prohibition on the Fae, when speaking to Harry, a person Uriel knows to also understand the lie-prohibition against the Fae, would equivocate on the definition of lie. That is especially true given the circumstance that Uriel was not attempting to mislead Harry but rather was attempting to educate Harry on a very important truth - that Harry still has free will even after becoming the WK. It simply doesn't pass the smell test.
Therefore, in my opinion, it was either a very minor and isolated incident of bad writing, or it was a clue that Uriel was using a proxy in a story where proxies were an important theme.