His feet were wrapped and his upper body was wrapped, there was no fighting for yards. He was down because his forward progress stopped. The time of the whistle also doesn't matter as long as the ref determined him down. Watch a game and pay attention to the whistle, it's not uncommon.
Actually... He was ruled "Down by Contact" by the official on the field. If he had been ruled down because of forward motion then the play would not have been reviewable. Since he was actually ruled "down by contact." the play was reviewed. At this point the only job of the replay official was to decide if he was indeed "down by contact" Which it is easy to see on the video that he was not down by contact before the ball was fumbled.
Therefore it was an easy call for the replay official to deduce that no the player was not down by contact, and we are going to overturn the call, and I have yet to find a person that would disagree with that diagnosis.