It's only illegal if the Steelers don't get an opportunity to substitute. I didn't see that play, so I don't know what happened after what you describe, but the officials have declared an official timeout to give the defense a chance to substitute (I believe that happened in the past couple weeks.)
But it's not illegal to rush your punt time out after lining up the offense, or vice-versa. Rather unproductive and silly, and it illustrates how much the Pats have fallen from seasons' past. But in and of itself, not illegal.