It's not about smart vs dumb, it's about intrinsic versus extrinsic motivation. Being smart is an advantage, but it is meaningless without a massive amount of practice. There's nothing about programming that a 100 IQ person can't become competent at it.
Programming is first and foremost, an exercise in frustration. It is exacting and unforgiving in a way that few human activities are. It's somewhat unnatural. In order to excel at it I believe you need to feel an intrinsic thrill when solving a problem that surpasses all the frustration leading up that point, and you need to seek this out over and over and over.
Note that this says nothing about whether you only work for money, or even what brought you to the field. However, if you do not feel this thrill, then you will naturally gravitate towards a job where you can execute rote incantations again and again without having to face new problems. I'm sure there are people who love programming and persist in it despite being terrible at it, but I guarantee you they are far outnumbered by the hordes of wannabe tech millionaires flooding into the Bay Area as we speak, completely oblivious to how much they will actually hate and subsequently suck at programming—and those people ain't learning Haskell.
This is more than two tweets long. How is it supposed to become the viral quote it deserves to be?
No, but actually bothering to learn something that isn't what is being shoveled through most major outlets is at least somewhat correlated with motivation.
No, there's no such rule. Can you possibly imagine that there might be any factors at play that affect the observed intelligence of Haskell programmers besides "a rule"?
I think you're confusing orthogonal issues. Only writing code because it pays the bills really says nothing about how smart or motivated a person is. Is there a rule somewhere that dumb people can't enjoy programming and learn Haskell? On the other hand I know a few very smart people who only write code because it pays their bills.