This was a key play on the final drive. I think Holmes went for 20 to set the Steelers up in great position.
Technically speaking the offensive lineman who is playing tackle is not at the line of scrimmage, and therefore there are not 7 players on the line, and this is an illegal formation. The guard next to him is similarly far back, but he's obscured by the tackle.
There should have been a 5-yard penalty.