Nah, that's just author over complicating simple thing (quite common among Haskell programmers). If he thought for a moment before starting Haskell, he would find out that associativity is enough for map reduce operation.
Sometimes, Haskell seems to be more mathematics than programming.
I found this interesting paper on the subject of non-commutative reducers: "Nondeterminism in MapReduce Considered Harmful?" https://www.microsoft.com/en-us/research/wp-content/uploads/...