I didn’t interpret the NTSB findings as a spontaneous explosion, but rather one with a clear cause.
IIRC they concluded that an electrical spark from a wire set off fumes in the tank, but there was no real evidence other than the the tank was the location of the explosion. I always thought it was just as likely that someone could have put a bomb in the tank. No way to know but I wonder how much they investigated the ground crew’s background.