I don't think this a fair interpretation of the parent comment as it's not about ownership of language. The website literally says "The next generation of ANTLR" and says "It's the successor of ANTLR4".

It's about a tool claiming to be the successor without seeming to be part of the ANTLR organisation. Are they completely different people, did the ANTLR4 owners stop writing it? There seems to be deliberately no clarification on this.