Its simple really. A spin putt imparts its energy on a horizontal plane. The disc comes towards the body primarily, which loads the wrist and elbow, and then extends straight out toward the basket. Any veritical motion in the stroke is primarily for feel, it doesn't actually do much to the putt. A push putt stroke doesnt come into the body. The push putt stroke is veritical, the disc is drawn down toward the pelvis and then extended up toward the basket. The stroke itself doesn't load the wrist or elbow. The elbow strays straight and the arm moves like a pendulum. Any added spin comes from finger spring or a little wrist load, both of which are completely unrelated to the actual putting stroke.
An actual "spush" putter would have to draw the disc both down and in and extend on both planes simultaneously. The only pro I can think of who actually does this is shuestrick, and I'm pretty sure nobody is studying his mechanics with the goal of emulating them. Trying to control two different planes of motion simultaneously is a terrible idea.