If the ball was snapped before the 11th player made it onto the field, then he has committed an illegal substitution foul, but nothing more.
Not that my opinion makes any difference, but I’ve never liked that rule. For the exact reason described in the given play, I’d prefer to see a rule that makes it a major foul for a substitute to enter the field while the ball is in play, enforceable at the end of the last run, with a first down.
Robert