"They had had a long battle with what looks to be hundreds of participants and there were a few wounded, but nobody died."
Observe the weapons. Most of them are sharpened sticks and they had a limited supply. Less deadly than swords and no mass manufacturing of weapons either.
The spears would be lucky to penetrate the ribcage. But they could go through the gut or throat, and if they were embedded in a thigh, they could easily cause sepsis.
You can easily kill someone with a fire-hardened pointed stick or a wooden club, if you really want to. I think it is just that they don't want to.
In the battle footage there is little or no hand to hand conflict, and the spears are thrown by hand (from what I remember): no catapults or sheathes to launch them from. At one point, someone removes a spear from his leg, and while it does look nasty, it doesn't look like a fatal injury unless sepsis etc gets in. I'm sure they did have battle clubs, but those aren't used much in this battle. (This may be partly because the battle is in rough/hilly terrain and it is easier to launch spears at people from a distance than run over to them.)
Guns began to enter the region in the 70s and 80s in large numbers, which apparently changed the dynamic. Today's Port Moresby, the most modern part of the island, is apparently much more dangerous than these areas were.
You’d have to catch them first. If you watch the video there almost zero hand to hand combat. Their arrows don’t seem to have stabilizers either.
Obviously if this was filmed on video someone could have given one side a machine gun. Would’ve they used it or would have they still opted to maintain the tradition and fight the old way. The battle may have other reasons besides 100% dominating or wiping out the opponent.
Sadly, guns have entered these conflicts since that film was recorded, and there have been far more fatalities. They've changed the dynamic and not for the better. Central government doesn't seem to have much control over big portions of PNG. Language and remoteness are two major barriers to that.