I heard that was directly due to development team deciding to rewrite their site in "the angular" because it was quote: "like Haskell but better"

Blew a huge hole in their search engine rankings.

Well, it does have really poor load times and isn't SEO friendly in the least. Where I work, we are going through the same pain but with a different -- more in style -- set of front end practices.

It's a real shame what poor management, idealism, over-engineering and developers desperately putting their own goals ahead of the company will do to a product.

How did rewriting the site in another language affect their ranking?

Angular is a javascript framework rendered on the client and javascript is not yet 100% supported by the crawlers at search engines (but it's getting better).