Except I said "fairly well", not "very well".
Texas held USC to 5.1 ypc on the ground, which is considerably below what USC averaged in '05. 5.1 ypc is still good, but Texas did a "fair" job of containing USC's rushing attack, holding them below their average, and assuring that USC would have to win the game through the air.
While USC's rushing offense only gained only gained 5.1 yards per attempt, their passing offense gained 8.9 yards per attempt - better than their season average.
Meanwhile, USC gave up 8 yards per attempt rushing, and 6.7 yards per attempt passing. The Texas offense found USC easier to run on then their average opponent, but harder to pass on.
USC at 5.1 ypc could not hang with Texas at 8 ypc in a rushing contest. So they had to pass to stay in the game. And they did. And they were able to pass effectively enough to move the chains with ease.
The lack of run defense of '05 USC was a crushing weakness against Texas. The '04 defense was stellar in this regard - far superior to '05. This has a huge impact on the matchup.
Rushing offense was not a strength of USC vs Texas. Their passing offense was the strength, and they had to rely on the pass because they couldn't hang with Texas running the ball.
For USC in that game, they do better if they sacrifice rushing effectiveness for superior rushing defense. This shores up their big defensive deficiency, without sacrificing their primary offensive weapon.