That is what I think too. The restraint was properly secured and did not malfunction, the problem was the body type of the person (more so than the weight even); in a regular drop it indeed seems there would have been no problems, basically there was too much of gap, this was ok while in freefall but once the breaking part comes and probably the seats moving into regular position again, those changes in force and momentum would have move him to the gap, I read something today of some ride expert saying in essence he was sucked out of the ride and how a seatbelt might have prevented it.
As for seatbelts, I have often wondered this, the way they are on a B&M coaster for instance, say the restraint breaks, would the seatbelt really work with all sorts of people's weight?