This is a great explanation. I've long thought about these two conflicting effects, and I think as you pointed out it has something to do with what nowadays gets described as 'culture'. Dunning-Kruger seems to dominate in the 'everyone gets a trophy' culture in which I was raised; whereas in cultures that (at least in the way they speak about themselves) pride themselves on meritocracy, leaderboards, stack ranking, and the like, the impostor syndrome probably takes over.
When alone, I compare myself to people in the videos I watch, People like Hickey, Carmack, Crockford, etc. I can't handle a candle to them! But when in BigCorp, intertia is massive, people souls have long been sucked out, and you feel shackled. This make you believe that you can accomplish way more than the median person (which is of course probably wrong).