Although it's very brief, they touch this topic in the intro: "OCaml does a great job of clarifying and simplifying the essence of functional programming in a way that other languages that blend functional and imperative programming (like Scala) or take functional programming to the extreme (like Haskell) do not. Having learned OCaml, you'll be well equipped to teach yourself any other functional(-inspired) language."
This course is for teaching students who know how to program but are new to functional paradigm. The way I understand the above is that OCaml is impure, so it's easier to have some parts in imperative style to get unstuck and move forward at first. That may reduce frustration for some students. Then over time, as the functional approach sinks in, you move to a purer style (with sometimes imperative parts, but well encapsulated). Haskell would be more like "let's jump at the deep end of the functional pool" --- which can work for some too.
This course is for teaching students who know how to program but are new to functional paradigm. The way I understand the above is that OCaml is impure, so it's easier to have some parts in imperative style to get unstuck and move forward at first. That may reduce frustration for some students. Then over time, as the functional approach sinks in, you move to a purer style (with sometimes imperative parts, but well encapsulated). Haskell would be more like "let's jump at the deep end of the functional pool" --- which can work for some too.