Cory Diaz is a big time shill for anything Louisiana. I suspect he's simply angry that Manning didn't give LSU a closer look.
While it's true that Manning's junior and sophomore seasons aren't as good as his freshman season, that's a rather skewed view on things, since he didn't attempt nearly as many passes as he did as a freshman.
He still had some really nice numbers, especially when it comes to keeping his INT's to a minimum, and he's a proven commodity in the high school football world. As far as I can tell he has a good eye for the game, excellent timing, very good accuracy, and a strong enough arm. Almost every FBS school wanted him, and that simply doesn't happen to a 3 star player with a big name attached.
To put it this way... He's going to do just fine at Texas, barring some freak injury or mental setback.