I have spearfished many big fishes ( 50, 60 kg ) in my diving days, and I would say that I find this very hard to believe for the mere reason that I don't see 1) that he had any mechanism with extra line tied to a buoy to let the marlin dive, ( but perhaps it was floating nearby..). 2) With a fish of that size everything, him and the video camera included would take a very quick dive .) I don't see where, shot where it was ( soft tissue ), that harpoon could stay in place for long . 4) How do 4 guys pull that size a fish out of the water, with that harpoon string?
Sounds fishy.. I could be wrong, tottally, but...