I don't think they are acknowledging any impedance mismatch there. The point of that paragraph is to emphasize that Haskell retains the usual feel of imperative

I don't think they are acknowledging any impedance mismatch there. The point of that paragraph is to emphasize that Haskell retains the usual feel of imperative programming, with the important difference that the imperative bits of your code are cleanly separated from the pure bits due to the IO type. There's no theoretical stuff here, it's just making sure that the caller knows about callees having side effects.