Pretty sure. Blocks in the back are legal on players in the box at the time of the snap, provides the ball hasn't passed put of the box (called the free blocking zone), but to my knowledge it's otherwise illegal.
Ok, I had to look it up. Here's what the rule says:
Rule 3, Section 4
A Block in the Back is a block that is delivered from behind an opponent above his waist. It is not a block in the back:
A- if the opponent turns away from the blocker, or
B- if both the blocker's hands are on the opponent's side.
Although I didn't say it as clearly, A is what I was referring to. Thomas had turned his back on Luke and was running away from him when Luke pushed him in his back. So, based on the rule, Luke did not commit a penalty on that play.