That's how the rule works, but looking at the play ….
1. Denver failed to get a first down.
2. Denver's line failed on the punt attempt and Denver got their punt blocked.
3. The Cowboys never took possession so it wasn't a fumble (although the rule treats it as such).
4. Denver got a 1st down without ever having to gain the yardage to do so.
Further, under this rule it wouldn't even take a Cowboy attempting to recover the ball for it to work out this way. For example, if the blocked punt shot off into a mass of players just across the line of scrimmage and inadvertently hit a Cowboy first then Denver could jump on the ball and get a 1st down by nothing more than pure luck. I'm not a fan of a rule that makes it possible to gain a 1st down without having to earn it.