There is little-to-no actor composition in this post, except that which is hidden by the framework that they've implementated... instead, it describes using the methods of an interface to communicate between your implementation, and their framework.

Composition of actors would allow for "wrapping" the actions of one actor in another and an implementation of RPC within your service itself (separate actors for both requests and responses), but that's not generally a thing you would do directly. Why? Because it's nontrivial, and you'd likely want a framework like theirs to orchestrate it.