The Java 9 API is based on the open source Reactive-Streams API that already existed. As of the time of writing the book, none of the projects actually implement the Java 9 interfaces directly. However, it should be possible in the future.
They are not directly interchangeable due to their different implementations. However, it is possible to build a library that exposes only reactive-streams API interfaces, such as Publisher but use RxJava (or something else) under the hood. In this sense they are interchangeable.
There are many considerations which are outlined in the book. Mainly they make different trade offs.