From your included snippet:
You highlighted the boarding/disembarking clause. But look at the part I bolded. Did that Taylorcraft suffer substantial damage? The comma and the "or" grammatically separates that condition out from the clause preceding it. Commas and conjunctions in legal verbiage are important. It would seem to me that the language specifies an "accident" occurs when any person on board the aircraft suffers death or serious injury between the time such persons board and disembark said plane, or when the aircraft receives substantial damage. In this case, despite the sole occupant having already disembarked, invalidating the first option, the aircraft has definitely received substantial damage. Thus, an aircraft accident has occurred.
Although, having viewed the obviously edited YouTube video--despite serious reservations about giving the poster another click--I would say, in my opinion, it was not likely an "accident," but by design. So maybe his defense hinges on the legal definition of "accident." Also, IANAL, just SGOTI.