I think that's a bogus assumption.
1. There is (was) a bulkhead behind the pilot that I feel would preclude him from sliding or falling backward.
2. If the shoulder harness system in the Galloping Ghost was an inertia reel type, then the shoulder straps would allow the pilot to slump forward after experiencing GLOC. If slumped forward, he would not be able to be seen by viewing the canopy from the side. In another photo showing the aircraft inverted, you can barely make out the pilot's helmet against the forward part of the canopy due to gravity.
3. That last picture, which supposedly shows the airplane in the last seconds of the dive into the ramp area might also be bogus. Note the reflection on the spinner showing the horizon (and airplanes on the ramp when viewed elsewhere on the net) aligned with the longitudinal axis. The reflected horizon in a dive should be perpendicular to that direction of flight if the aircraft was pointed to the ground.