My understanding is that because the ball was hit out of his hand while he was bringing it forward it that makes it a dead ball. If he had just thrown it backward and it hit the ground it would be live.
That tuck rule is atrocious. If the QB doesn't get forced fumbled, it's either a sack, intentional grounding, or fumbled lateral. It's like rewarding the offense for doing something wrong.
What's even worse about that call is you reward the offense for a terrible decision and punish the defense for making a great play. Completely backwards.
Even if he was throwing a pass....which he wasn't, how come the ball came out backwards? Still a lateral. Total BS.