Momentum alone did not take the player over the line. He promoted the ball with a second intentional movement after the ball carrying arm had hit the ground.
The confusion seems to be coming from the fact Sam Tomkins still had momentum which would have resulted in the ball and player ending up infront of where the ball carrying arm hit the floor but the issue is that he assisted that momentum in progressing the ball forward with a second movement which he is not allowed to do after the ball carrying arm has hit the floor.
For instance is a player allowed to pass the ball after the ball carrying arm hit the ground? - no, and we've all seen tries disallowed for doing that. I'm surprised it was allowed to stand and I think it was because it was so done so obviously confidently that people questioned their own judgment. Anyway it was awarded and it did take some scoring.