Brock struck Roethlisberger near the right knee right after the quarterback delivered a pass to the other side of the field. That much, everyone can agree upon.
Whether or not Brock committed a foul is open to interpretation. I've gone through the rules on roughing the passer and can make the following observations:
Brock did not violate the "one-step rule" preventing defenders from taking more than one step and striking a quarterback after the ball was released. He barely took any step at all. His right toe and knee hit the ground before the ball had traveled even 3 yards from Roethlisberger's hand. There were no additional steps before contact.
Brock might have violated the rule stating, "A rushing defender is prohibited from forcibly hitting in the knee area or below a passer who has one or both feet on the ground, even if the initial contact is above the knee. It is not a foul if the defender is blocked [or fouled] into the passer and has no opportunity to avoid him."
That last part -- whether Brock could have avoided Roethlisberger -- is open for debate. Once Brock beat Gilbert with the spin move, Gilbert extended his left leg across Brock's path. It appeared to me as though Gilbert, knowing he'd been beaten, might have been trying to trip Brock. That would be tough to verify.
When in doubt, the NFL will err on the side of protecting quarterbacks, especially big-name ones.