The Wahgi people were isolated from the rest of the “modern” world until 1933. They came into contact with WWII service men who shared some aspects of western culture with the tribesmen. In particular, they showed them the comic books they read while shipped out. The Wahgi loved them. In particular, the Wahgi adored the stories of the Phantom, who wasn’t even particularly popular in its home of America.
He is so popular that the few Wahgi who can read english will read the comics out loud in the village center and hold out the pages for everyone to see, so the whole tribe can enjoy them and marvel at the Phantom’s might in battle.
They identify with the Phantom because he came from a jungle territory, like them, wore a mask to fight, like them, and came from a long line of warriors, which the Wahgi, who worshiped their ancestors, deeply respected. Further, despite not really having superpowers, the Phantom is strong, clever, and incredibly fast. He was so fast that his enemies began to believe that he was impervious to bullets and could not be killed.
Therefore, the Wahgi began painting HIM on their shields to invoke HIS abilities in battle. There are TONS of Phantom-Wahgi shields out there.
So, you might think that you’re huge comic book fan, but the Wahgi have taken their Phantom fandom to the next level and have made the Phantom a fucking talisman to carry into battle for strength.
"Because the Phantom is cool" - Old Jungle Saying