In short, "unless his strength is tripled" means "unless the character he is skirmishing has strength at least triple his own."
Normally, a character is overwhelmed if his enemy has strength at least double his own.
So the strength 6 dwarf wearing the
Dwarven Armor can be overwhelmed if the minions he is skirmishing have a total strength of at least 18. Without the
Dwarven Armor, the minions only need strength 12 to overwhelm.
I agree that the wording is ambiguous; they should have made it a loaded keyword in my opinion.