Based on the NFL's weird definition of what is and isn't a catch, I think it's a legit catch. I believe the rule that applies here is:
The bolded part makes it tough to say whether or not it's a catch. To me, since a slight movement is NOT considered losing control of the ball, I would say that losing control of the ball requires the ball actually leaving your hands for a moment (bobbling the ball). Looking at the Bryant catch, I don't think that ever happened. It moved around in his right hand a bit, but he always had his right hand on the ball and had it pinned against his left hand or his leg at all times. So since he never lost control of the ball, the three steps that he took in bounds count and it's a touchdown. Even if you consider the part where the ball moves from his two hands to his leg as him losing possession, he pins the ball against his leg with his right foot on the ground, gets his left foot down, and then does the flip.