Error message was, indeed a typo on the post, not on the code.
Thanks for the answer but I'm afraid you lost me on there...
This is my configuration for compiling that particular file on Maven:
I don't think I understand what you mean by "generating the Java 8 class file format". I would expect that maven configuration to produce bytecode for java 8 and thus, the interface bytecode to have the default implementation. Is there anything that needs to be configured in the kotlin counterpart to be aware of it, maybe to be able to say "when using that clase, note it is Java 8 and treat it as such"?