I think they're saying that it's counter-intuitive that the specialization relationship between Fn, FnMut, and FnOnce is the reverse of foo, &mut foo, and &foo.

I think they're saying that it's counter-intuitive that the specialization relationship between Fn, FnMut, and FnOnce is the reverse of foo, &mut foo, and &foo. I know it took me a long time for that to stick.