This is true, but they were depending on the defense to get it wrong despite the number being announced. I don't really have a problem with that. If the number is announced and the defense covers the wrong player, it deserves whatever happens. But it is the offense's responsibility to make it clear to the ref who is reporting, and that's where Detroit tried subterfuge and it failed.
If 68 had been the only player to go up to the ref, say he was reporting and making the appropriate gesture, then there would have been nothing wrong with Detroit trying to fool Dallas by also bringing in the player who had been reporting eligible throughout the game to get the Cowboys to think he was the eligible receiver again.