To paraphrase: a delay-of-game penalty is called a "dead ball foul," which means the game stops before the ball is snapped. So in the case of the Lions FG attempt, the delay of game nullified anything that happened after the ball was snapped -- the missed FG basically never occurred. So there was no play that could be accepted.
Same goes for the punting situation -- once the delay of game is called, there is no punt, no play that could be accepted. It's either accept the penalty or decline the penalty, and replay the down.