And going by his other papers, though, it looks like the effect isn't nearly so strong as was originally claimed. (Though that's wrt whether his "consistency test" works, didn't check about whether bimodalness still holds.)

No, just personal experience and observation backed up by stories and blog posts from other people. See also Joel Spolsky on Hitting the High Notes. Spolsky's line is that some people are just never going to be that good at programming. I'd rephrase it as: some people are just never going to be motivated to spend long hours programming for the sheer fun and challenge of it, and so they're never going to be that good at programming.