[coolsunglasses]

>Will these roles involve working with a non-standard Haskell compiler?

They have a pretty well known internal compiler for a strict dialect of Haskell.

>What level of Haskell experience are you looking for ... what are some milestone Haskell skills that could help someone quickly identify whether they are plausibly a good fit?

They're unlikely to answer this and I don't work there, but you should probably be able to reason about things like GHC's RTS as a baseline.