I'm not all the concerned about his shot as long as it's a high, quick release and it goes in. Steph Curry shoots on the way up on his jump shots and Shawn Marion really had a different stroke. The step back two reminds me quite a bit of Malcolm Hill.
Shooting on the way up is proper mechanics. Shooting at the apex of the jump is bad mechanics, regardless of ancient dogma. When you get to the top of your jump, all the energy transfer comes from your arm and hand. It makes it much harder to get enough power to shoot deep threes: you end up using small muscles that are more difficult to coordinate consistently.
The old methodology around shooting technique is virtually all wrong. I'll describe the worst way to shoot: square your shoulders to the basket, keep your elbow in tight to the body, bring the ball straight up to the shooting pocket (no dip), jump high (get good lift) and don't kick your legs, shoot at the top of your jump, extend your shooting arm and get a high release, follow through by making a gooseneck with your shooting hand. The WORST.
Proper/new way to shoot: angle your body naturally to the basket (around a 45-degree angle is usually good), bring the ball to the shooting pocket in one fluid motion (it helps to dip the ball slightly at waist/abdomen level as you flex at the knees - this creates more power), transfer kinetic energy from your jump into your upper body by shooting early in the jump, keep your release relatively low (a high, straight armed release is energy inefficient and requires you to engage dozens of small hand and wrist muscles - you want 90% of the power to come from your legs). Most important thing is consistency, and consistency is easiest to achieve when you eliminate variables. Steph Curry's shot looks "weird" because he's breaking a lot of dumb old rules in favor of a more efficient, streamlined technique.