Maybes there's something in having to be brilliant in general but idk about this blanket statement. At least when I have had engineers turned manager as my manager, a lot of the time they seem to not be able to turn off the engineering. There's always something to tweak or be hands on with or experiment, except its not code they are messing with, but people. Our team structure, meetings, lets measure stuff, change our process etc.. and it ends up being a distracting drain. Engineers don't seem to be able to not mess with things. But maybe that goes with brilliant? Brilliant engineers know when to tweak and not to.

But that doesn't seem fair, yes brilliant people will always be good, you can't count on having brilliant people