Your argument is that a language with a 750,000 line implementation and a zillion features is simple because one of its IRs is simple? Doesn't seem very convincing.
But I'll leave you to argue with Vitaly Bragilevsky (speaking at Galois):
"Haskell is a big language" -- https://galois.com/blog/2018/11/teaching-haskell-in-the-real...
Or Paul Hudak, Philip Wadler, and Simon Peyton Jones:
"Haskell is a big language" -- https://www.microsoft.com/en-us/research/wp-content/uploads/... (p.28)
Maybe those guys know something.
It's cool Haskell goes through a small IR. That doesn't make TH, rewrite rules, CPP, the million extensions, STG, the runtime, the gigantic syntax, or the quarter million SLOC implementation simple.