Now that I think about it, I'm absolutely certain they changed the rule because I can recall a play from last year when the ref specifically mentioned it.
In the Miami/Pitt game when Ben was awarded a TD, but it turned out he really fumbled the ball. After reviewing and determining that it was a fumble, despite the whistle having blown, Miami would have been awarded the ball if it could have been proven that they were the ones who recovered it (the ref specifically said this). Without evidence of recovery, the ball was awarded to Pittsburgh.
Unless the rule was changed back this offseason, the refs in Denver were both incorrect in their call and in their challenge interpretation.
I don't think that the rule was changed back, so the refs in Denver really DID blow it twice. In the DET-NOLA game, though, I thought the whistle came AFTER the Lions had clearly taken possession. So, either way, while DET ended up being deprived of the return (potentially for a TD), once it was determined to be a fumble, it was the Lions' ball - and would have been even without the rule change. I think.