I understand that the slipper will still work, but if they pinned the rear shaft to the front pulley, the front and the rear cannot move independently. what slips is only the spur between the two fixed slipper hubs.

Currently the rear hub is indexed to the front pulley. The drive comes from the pressure on the thrust washer from the whole slipper assembly. decoupled, hence the problem.

The outer hub is indexed to the shaft and drives the rear w/gears.

Unless they did something totally different from what you described, it would not be decoupled anymore