That would not run counter to the popular (whether true or not) idea that by using functional programming languages you filter for a higher quality labor pool / applicant pool.
That wouldn't apply here, since as the article says they hire "generalists, and most of them have never written a line of Haskell before joining."
In any case, I think the "Haskell tax" concept (where you can pay well-paid programmers less if you have a Haskell shop) is stale by now. Rust attracted away a lot of FP-ers, plus mainstream langs like C++, Java and even Typescript got smarter. Haskell's biggest problem by far is the tiny labor pool, which Mercury seems to wisely avoid.
The version I've always heard is just well designed but less popular languages, but the ones I can think of were all functional (Haskell/F#/OCaml/Clojure/Elm/Erlang)
