I hear what the author is saying, but I think if composability is your goal, you should look no further than traits/interfaces. Accomplishes the same goals but actually far more general (which, according to the author, is a good thing)
Trait and interface systems use a subtyping relation; they aren't alternatives to a subtyping relation.
I think subtyping is the general concept in the programming language / type literature and interfaces / traits are an example of subtyping. If A implements interface B, A is a subtype of B.