Even if Haskell was objectively the 'best' language, it'd likely be a poor choice for most teams simply due to familiarity and developer speed.
I'd be extremely hesitant to hire the services of this company entirely because they use Haskell and it'd be a nightmare to maintain after their contract.
From my reading the article makes no claim that Haskell is the best language. The purpose as I read it is to explain why Haskell is their first choice while addressing an audience that has a passing knowledge of Haskell.
You being extremely hesitant to hire the services because of their technology stack is great and fine. There are many stacks and services I completely avoid in dealing with so I agree there.
"It'd be a nightmare to maintain after their contract" is something someone would likely say to any language that is not their preference.