You’re missing three things. First, the receiver was never touched by a defensive player. So the ball was NOT dead when his knee touched the ground. Second, if you watch the video, the ball touched the ground. And as soon as that happens, it’s considered a factor in his getting control of the ball which instantly negates the catch. Third, the precedent for defining a “football move” has been to take steps while maintaining control of the ball. He took no steps after the ball contacted him. Keep in mind that this rule was set up the reduce the number of fumbles that were occurring when receivers got hit by a defender (or in this case, the ground) before the receiver could make a “football move”. That call was not confirmed by the officials on the field. It was confirmed by the NFL review team ostensibly consisting of the best officiating minds in the league. With the advent of incredibly precise and revealing video capabilities, officials are required to parse plays literally to the millisecond. As long as the rules are applied consistently, no one should have a beef.
“He took no steps after the ball contacted him.”
He technically became a runner when he made his football move by stretching for the goal line.