Illegal Formation or Illegal participation?

[keystoneref]

This isn't a foul, he fulfilled the requirements of being within 9 yds of the ball after the ready for play and before the snap. 7-2-1 "AFTER the ball is ready for play, each player of A who participated in the previous down and each substitute for A must have been, momentarily between the 9-yard marks, before the snap." If all A players had to be within 9 yards of the ball, either before or at the time of the ready for play signal, how would you ever make a legal substitution after the ready for play?