I can definitely vouch for that, however not in the absolute official battle.
They grab two wild bettas. They bet on them. They allow them to fight, and usually they will last quite some time (they seem to be more vicious than our domestic bettas!) before they slow down. Then it becomes ...*nip* ... *NIP NIP* ... *nip*. And at this point, one attempts to retreat - or ceases fighting, and the bet ends. Unless both bettas are still nipping each other, it will end. They usually do not fight to the death, but they have had deaths anyway when it came down to two really tough ones.
Sometimes there are "referees" (in their case, it's a parent, LOL), but normally it's just the cousins / friends coming to an agreement on whether or not who wins. If they still don't agree, it continues.