I wondered if somehow he charged up one of those humongous phase-correcting power line capacitors and dumped that into the melon. He'd have to rectify it to DC first, though. I would guess if he had that xfrmr in the yard, he had access to other HV stuff, maybe HV rectifiers, too.
In some spark-photography experiments I did years ago, I used 16 1kV silicon rectifiers (1N4006) in series to charge a couple of caps from a 10kV oil burner ignition xfrmr. (10kV x 1.414 = 14.14 kV peak voltage.) But those were only one-amp units.
The crack from the spark sounded like a .22 rifle, at least in my garage.
