I think Frost knew right around the time of the meeting in Philly, so I can sympathize with the kid on that front. But at the same time, whether Frost knew or not, it was his responsibility to work in UCF's best interest, not NU's. It was a catch-22 moment for Frost. Damned if you do damned if you don't.
It might have just been a slip of the mind on Frosts part too by phrasing it that way - unintentionally painting himself into a box. I talk to a lot of different clients/customers at work, and when I'm tired and client asks me a point blank question (that answering sometimes violate other clients' rights), sometime its tough to phrase an answer that isn't construed in the wrong way - especially when one is naturally honest and candid with people like Frost seems to be.... I can imagine Frost having some troubles with that aspect of everything. I doubt he meant or wanted to lie to recruits, but he couldn't just spill the beans to a 17 Y/O kid only for it to blow up on Twitter.
These parents need to do a better job of explaining that part of life, because everyone, even Eriq, is put in a similar situation catch-22 moment at some point in life.