This isn't surprising, really, but not from a 'I am smart, everyone else is stupid!' sort of perspective.
The rationale behind Brexit, rightly or wrongly, was something that appealed to working class, blue-collared voters, etc. Bringing jobs back to the UK, lowering immigration, etc. It didn't have as much appeal to the middle / upper class, white-collared voters, etc.
You will find correlation between class, job and educational attainment. You will also find some correlation educational attainment and cognitive ability. Ergo, the correlation between a persons vote and cognitive ability isn't surprising.
Leave voters were misinformed and misguided, but I don't see what good it does anyone to call them "thick". I know a lot of people who voted leave but are otherwise intelligent and very capable. If anything, I think the attitude that leave voters are all "thick" is what pushed a lot of people into voting that way (particularly the people who felt ignored by the 'liberal establishment'). Insulting people doesn't win them on your side, least not at a country wide level.