Official Google Actions SDK is written in Node.js. But in many situations voice interfaces like Google Home or Google Assistant will extend or replace mobile apps. If you are old fashioned Android engineer and the most of your code is already written in Java, why not reuse it and build voice extension to app on your own? And this is the main reason to build Google Actions Java SDK - enabling as much developers as possible to build their brilliant ideas for Google Assistant and Home.
Currently this is just working proof of concept of Google Actions Java SDK. It means that there is no documentation, fixed interface, (not much) unit tests and many, many others.
Google Actions Java SDK is build based on official Node.js library, but it's not a mirror copy of it. The goal is to make it fully compatible with Conversational Protocol of Assistant Platform.