You appear to have missed where RMS was supposed to have appointed a new co-maintainer without asking anyone in the current team, therefore asserting ownership of a project he was uninvolved with in what appears to a spiteful reason related to his speaking out against RMS. It's in the bottom of the message. It's weird that your response is to threaten to avoid something you've never known about before. Unless you were active in the GNU community already you wouldn't have had reason to use it, so you've not created GNU extensions, were unaware of the language and the threat you're making is that you'll avoid it.

Regardless of whether the statements are accurate, I'll hold judgement until hearing the other side though it does seem odd. I'd be curious what the prospective co-maintainer wants since he quit prior without resolving issues.