All three teams had access to the Rules of the FA Cup competition, which they should have made themselves familiar with, all three teams confirmed (incorrectly) that they were allowed a sixth substitute in extra time prior to the game.
One or all of them should have challenged that.
Exeter did execute the sixth substitution, Bradford attempted to as well but, for an unknown reason, were prevented by the 4th official from doing so.
They were all complicit.
That's horrific and going too far in every sense!