I'm only bi, but this isn't obvious at all to me. In fact it seems obviously wrong. Most marginalized groups don't "overcompensate in achievement-related domains", and I don't see what mechanism would make LGBT people be any different.

More plausible to me is that these studies can't capture the true concentration of gay people, only the concentration of out gay people. And it's easier to be out, to the rest of the world and to yourself, when you're in an environment that won't punish you for it. And richer environments tend to be less homophobic, so even if the concentration of gay people is the same, more will be out in the richer parts of society.

I grew up in an very homophobic, very poor rural community, and convinced myself I was straight. Then when I went to college I had basically my first exposure to openly gay people and eventually realized I had been deluding myself. That would probably never had happened if I stayed in my hometown.