Support for Markdown-based JavaDoc has been available since at least 2009 via multiple implementations [1, 2]. There have been many extensions thanks to the doclets api. The difference here is that Markdown will be included in the standard doclet and supported by the Java team instead of a 3rd party. Kotlin has few innovations and is rarely a source of inspiration for Java changes, e.g. Jetbrain's often called Scala a research language that they borrowed heavily from to design an industry language.

[1] https://code.google.com/archive/p/markdown-doclet

[2] https://github.com/Abnaxos/markdown-doclet