Specifically, Louis had a Sling and the two remaining Sibling players had Spears. Sling is invulnerable to Spear and always beats it. So over the course of three rounds, Louis was able to pick-off the Spear-wielders.
Weapon selection is supreme. Number of participants is important, because it means your tribe has more weapons in play and can absorb more damage, but ultimately weapon selection prevails over number of participants.
Here's an extreme example:
Tribe A has one player, who was a Spear.
Tribe B has twenty-seven players and for whatever reason all choose Shields.
Tribe A wins because the Spear is invulnerable to the Shield, but the Shield is vulnerable to the Spear.
This balances situations where one tribe is smaller than the other, and gives a smaller tribe a fighting chance.
It may seem counter-intuitive, but keep in mind that the rocks-scissors-paper system of resolution is only a colorful device to determine which tribe wins immunity in the first half. The actual game is won or lost in Tribal Council votes.