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.
Different people learn differently. Something that is obvious to you might not be obvious to someone else. The author learnt in that way and shared that, which is a good thing.
Sometimes, Haskell seems to be more mathematics than programming.
However some map-reduce implementations implicitly expect reducers to be commutative, which the author became appropriately suspicious of because of the "typical examples" of map-reduce.
Sometimes, Haskell seems to be more mathematics than programming.