According to the NFL's referees, Cardinals TE Darren Fells did not catch this ball, and it was an incomplete pass, not a fumble.
The Seahawks really wanted this to be a catch and fumble. After the hit by Bobby Wagner, the Seahawks were the only players around to scoop the ball up. They almost got a fumble and a shot at points before the half. Instead, this was incomplete, and the Cardinals kicked a field goal.
The rule in question is the same one that came into play earlier on an Odell Beckham Jr. touchdown that wasn't actually a touchdown, a ruling that changed the course of the Giants-Patriots game. A receiver needs to control the ball through the point in time when they "become a runner," which is defined as the moment "is capable of avoiding or warding off impending contact." This wording is new this year, replacing the dreaded "football move" language that previously existed.
On the Beckham touchdown/non-touchdown, it was clear that moment never arrived. Although it's weird that players have to "establish themselves as runners" even when in the end zone, where they don't have to run anymore, the refs made the right call there according to the rulebook.
Fells caught the ball, controlled it with both feet on the ground, switched it to his right hand, and began running upfield. But apparently, he wasn't "established as a runner." The rulebook doesn't quite explain why this wasn't a catch. This might've been a bad call on top of the confusing rules.