Problem was, while explaining the call for the media, the crew chief Ted Barrett said he "flipped" the ball from his glove to his hand. To me, that means he had voluntary release. I don't think you can have a flip without having voluntary release, even if it is dropped. On a side note, I haven't seen the play, only read about it.
Umpires defend call in 8th inning - Extra Bases - Red Sox blog