This document seems to be at a lower level. An actor model would be implemented in terms of these primitives. These primitives are available in many languages.

Immutable is kind of irrelevant in a discussion of multi-threading because immutable data has no problems in this regard. Which, of course, in a way also makes it relevant because one prevents issues. Ultimately one cannot have just immutable data because the computer would not be allowed to do anything. This document would be about data that cannot be immutable.

But the complaint sounds a bit like an article about assembly language getting the complaint that it does not explain how to implement a word processor.