I think this is a very good point, and the big issue. If Diana is born exceptional, the contest is rigged. If Diana is equal to the other Amazons, what makes Wonder Woman special when compared with the others? And as Agent Z says, the contest says nothing about Diana's character as a hero, just that she's skilled.
The only way around it I can think of is to change the nature of the Contest. I'd make it the Kobyashi Maru for Amazons. Any Amazon who wants to leave the island has to enter a chamber and face the Contest, effectively a compressed simulation of man's world and the dangers they'll face, and the overwhelming probability of failure. The test is a trap, a rigged game by the Gods. There's no way to "win". Man's world - the world in general - is a constant battle against an endless dark.
But Diana takes the Contest over and over again. Because she's the only Amazon who knows that the fight is what matters. The effort. The trying. And every Contest, she gets a little farther, does a little better, because the only person Diana has ever been in competition with is herself, who she was yesterday. Diana isn't trying to be The Best, she's trying to be Better. And that's why she's Wonder Woman.