No one's saying that (I think). The claim being made is that someone who wants a job programming in Haskell is likely to be someone who's interested in learning

No one's saying that (I think). The claim being made is that someone who wants a job programming in Haskell is likely to be someone who's interested in learning new things, not that someone who has learned some Haskell is likely to be a good programmer. Still, the thing linked is distinctly light on evidence (not a fault -- it's only an email on a Haskell mailing list) and I'm not quite sure what it's doing on the front page of HN; it's not exactly news that people wanting jobs programming in languages that are (1) difficult to learn, (2) interesting and (3) unusual, tend to be people who are interested in learning new stuff and keeping their brains active.