Because the play was well underway, Bortles already completed the dropback and was in the motion of throwing before the flag even came out. There was no whistle, it wasn't blown dead, they missed the call, it's essentially going "Oh wait, I forgot there was holding on the last play, I forgot to throw the flag, back up 10 yards please" They fucked up, and tried to cover their asses instead of letting the play continue like it should have.