My condolences to all who have been affected by this tragedy.
No speculation here but I will add that when the same malfunction happened to Bob Hannah in Voodoo, he did not have his shoulder harness locked. When he regained conciousness at 9000-10000 feet, he was bent over at the waist and his face was planted against the floor of the cockpit. At the time, Bob was in his prime and in excellent shape as prior to air racing he made his living as a professional athlete racing motorcycles off road - and was US National Champion for a number of years.
An almost instantaneous application of 9-10g's is a game-changer for everyone. I don't think Jimmy ever saw the ground fill the windscreen.