I honestly think a good amount of legibility is a matter of familiarity. I've used Haskell for a couple of years, and I didn't find this code particularly illegible (if I had to guess, this is probably more easily-understandable to me than the equivalent OOP formulation).
That's not to dismiss your criticism, not at all; rather, I think that the challenge would decrease with experience.
To underscore the point that legibility is as much a matter of familiarity as anything else: I think a big difference with an "OOP language" like the above poster seems to prefer versus Haskell has a lot more to do with the English language keyword operators that most contemporary OOP languages favor versus Haskell favoring more mathematical notation.
There exist "OOP Languages" like OG Smalltalk, Self, Io that might make for better comparisons to Haskell by virtue of being closer to the simpler syntax and general focus on more "mathematical" operators. Just as there exist more functional languages that use much more of an English keyword approach than Haskell.
(Some forms of Lisp/Scheme are so English keyword-forward with micro-DSLs that they may be a better example in a lot of posts like these for comparing "contempory OOP languages" and "functional languages". Plus there's all the bits of hybridized "functional languages" embedded directly inside contemporary "OOP" languages such as LINQ syntax in C#. It's interesting to me how many of these sorts of articles jump straight to Haskell.)
That's not to dismiss your criticism, not at all; rather, I think that the challenge would decrease with experience.