TTAG got the wording precisely correct - The
purchase was not legal because the purchaser lied on the 4473 about having been previously involuntarily committed. However, the
sale was legal as the pawn shop FFL ran the NICS check as required, and NICS didn't manage to catch the lie on the 4473.
The whole NICS system is broken. In fact, it started out broke, as it has never verifiably worked to prevent or deter a single crime, and of the many thousands of lies, omissions and otherwise illegal attempts to purchase guns through law-abiding FFLs and/or rejections from being caught by NICS checks, only a fraction of a percentage point have ever been prosecuted.
No one has anything to fear from NICS, least of all criminals. Actually, law-abiding citizens have much more to fear from NICS than criminals, because they can be denied on a mistake, and though they'll never be prosecuted for whatever issue they were mistakenly denied on, it is a time-consuming and (sometimes) costly process to get one's name removed from the list. Criminals know going in that they may be denied, so chances are they would never spend the time or money to get their names removed. Obviously, with the lack of prosecution for the felony of lying on the 4473, and the burden borne by LACs to get their names removed when mistakes happen, the law was never intended to work. It's nothing but a worthless hoop that LACs have to jump through with zero net gain to the safety of society at large. 'Murica. Ain't it grand?
Blues
