I kind of agree with this - if you want to do JavaScript, do JavaScript... if you're going to bother transpiling to non-standard JavaScript - go all the way and get the power of Haskell!
They did mention in the article that PureScript was one of the languages they considered rewriting their stack in. PureScript is very heavily influenced by Haskell (they're almost the same languages), but among other things PureScript is strictly evaluated.
It seems from the article like the reason why the dismissed it was that it interoperated poorly with existing vanilla JavaScript libraries.