They don’t use Haskell, but Mu. Which is a strict “dialect”, but with compatible syntax - how much and with which extensions I don’t know. So they actually removed the feature for Haskell’s existence (that’s like Rust without the borrow checker).
Mu has a strict runtime, which makes program performance easier to analyse and predict – but prevents us from just copy-pasting some Haskell libraries that rely crucially on lazy evaluation.
I’ve found this interview about Mu:
IMHO using them as an example for Haskell usage is a bit disingenuous.
I also find that funny:
We also have recursion disabled by default (simply because, unlike GHC, we have not optimised the compilation of recursive functions to avoid stack exhaustion) – but we actually don’t need it all too often, and we have all the usual recursive combinators (properly optimised) anyway.
So, loops or gotos 
But really, they didn’t succeed to do tail call optimisation?!