Nope, the OP is correct the rule states that the defensive substitutes are supposed to exit and enter the field in an expeditious manner (I believe the exact wording is "at an athletic pace" - i.e., running, not walking slowly). The Rulebook says the Defense has an opportunity to substitute after the Offense substitutes, but that is not what is being asked about here. The Rulebook ALSO states the pace that those substitutions should enter and exit the field at. The USC substitutions were both walking on and off the field - in fact, the substitute walking into the field of play didn't even start walking onto the field of play until the player leaving the field of play had slowly walked all the way to the sideline. That is against the Rules and the Official should have stepped away allowing PSU to snap the ball causing a penalty (or to few players) to USC as USC was abusing the substitution protocol specified in the Rulebook.