In nutrition it's count your calories, no actually don't eat carbs, or avoid saturated fats, or animal fats are fine, just trust your instinct, instinct is what makes everyone fat, etc.
The world of posture seems awfully similar -- everybody thinks they know what good posture is (which is often overexaggerated and incorrect), but few people can really define it, and different coaches have wildly different definitions for it (tilt the pelvis forwards! no, never tilt the pelvis forwards!).
But at the end of the day, the people who most depend on their posture to make a living -- actors, singers, and dancers -- overwhelmingly use AT. So, scientifically proven? Not at all. But it seems to be the best we've got.