Even if that were true, which I don't believe, it's still highly useful. E.g. if someone passes a GRE Physics test, they may be able to enter a graduate program with an entirely unrelated undergraduate degree instead of spending 100k getting another.

You may be different, but I'd have increased confidence in someone's general comfort with mathematical problem-solving if they posted good scores on a test covering this: https://www.ets.org/gre/subject/about/content/physics/