He's right to the extent that it's always very difficult to find a starting QB; therefore, it is inherently unlikely that a great QB prospect will fall out of the first two rounds. But this assumes that a player has to be a starter to have value. Getting a decent backup QB on a cheap four-year contract is worth it for a lot of teams. And if he plays well, the backup QB might accrue trade value, like when the Patriots flipped 7th Rounder Matt Cassel into an extra 2nd Round pick.
Russell Wilson is the best young QB in the league today. Nick Foles, another 3rd Rounder, had the highest QB rating in the league last season. Mike Glennon, yet another 3rd Rounder, was probably the best rookie QB. Kirk Cousins, a 4th Rounder, outplayed RGIII. Glennon and Cousins may eventually have some trade value for their respective teams, and in the meantime, they are capable backups on the cheap.
Of course, not every QB picked in the 6th Round is going to turn into Tom Brady. But in many situations, it's safer to take a QB in the later rounds than to waste a First or Second Round pick on a bust. I would rather take a chance on a Taj Boyd or James Franklin in the 6th Round than spend a high draft pick on someone like Derek Carr.