You are both partly right
Let’s try an analogy with Groovy.
Groovy.py is a Python implementation that supports Groovy 1.x (the language) and Groovydoctor is a Ruby implementation that supports the latest version of Groovy (still the language).
It’s the same with AsciiDoc (the language) with AsciiDoc.py (legacy Python implementation) and Asciidoctor (current/reference implementation) that supports the latest definition of the langage to date.
The AsciiDoc.py project will (soon) move out from https://asciidoc.org/. This website will become the homepage of AsciiDoc the language.
@melix If you are interested in working on a Java implementation of the AsciiDoc language, you should reach out to the AsciiDoc WG.
A first step would be to send an email on the mailing list: Mailing list: asciidoc-wg (121 subscribers) | Eclipse - The Eclipse Foundation open source community website. to tell that you are interested in contributing on the Java implementation.
You can join as an individual contributor or as a Gradle employee (if Gradle wants to join the AsciiDoc Working Group: Explore Our Members - Eclipse AsciiDoc | The Eclipse Foundation).
I know that some people working at VMware/Pivotal have also express interest in working on the Java implementation.
For reference, we will be working on a Java implementation as part of the Eclipse specification process (i.e., as an Eclipse project).
I hope this clarify things!