He quoted Heinlein in a way that made clear he regards himself as one of the 5% who can think and do - that seems narcissistic to me.
He clearly is one of the 5% that can think and do, as am I and as are you. I don't really think it is narcissistic to have a realistic opinion on some ability you might have.
I don't think Heinlein is 100% right. I think the 5% varies depending on the subject.
As to Brexit, surely, as with all political decisions it's about what you judge as important to you? I know lots of people who voted for it who are in any sense I understand as intelligent. Therefore it seems to me that dismissing those who vote for it as stupid because it's a decision I disagree with is highly questionable.
Nobody is dismissing the people who voted Brexit as stupid, just that they got the decision wrong.