I mostly agree with the article, but not with the HN title (which doesn't match the article's title). The article itself has a nice statement:
> If you’ve already learned another language, you can learn Haskell. And even if you haven’t, learning Haskell is no harder than learning any other programming language.
But most of the programmers are familiar with imperative languages, and can quite easily switch between them (possibly writing non-idiomatic and awkward code, but without having to really learn the language from scratch to do so). While a purely functional language with unfamiliar syntax doesn't let one to do it as easily. So indeed, if you haven't learned other languages, probably it's not harder, and if you have, you still can learn it, but likely it would be harder to learn than other imperative languages if you are already familiar with a few imperative languages.
> If you’ve already learned another language, you can learn Haskell. And even if you haven’t, learning Haskell is no harder than learning any other programming language.
But most of the programmers are familiar with imperative languages, and can quite easily switch between them (possibly writing non-idiomatic and awkward code, but without having to really learn the language from scratch to do so). While a purely functional language with unfamiliar syntax doesn't let one to do it as easily. So indeed, if you haven't learned other languages, probably it's not harder, and if you have, you still can learn it, but likely it would be harder to learn than other imperative languages if you are already familiar with a few imperative languages.