When I read that example the semantics were obvious to me: adding a map (applying a monoid operation on a map) should add the elements (apply the monoid operation for the elements).
And yes, you can argue that using "+" implies commutative, and I can see that, but I think it's still a plausible meaning in this context.
(I'm probably rabbit-holing here, but it's interesting.)
The usual implementation technique is type classes, and the issue of having multiple type classes (implementations of, say, monoid) for a given type is known as "type class coherence".
In Scala you could just pass a type class instance explicitly. If Haskell you'd have to change the type.