So, Gruber is fine with people implementing their own markdown processors without his permission, and still call it markdown, and he even likes it when they do that, he likes the flexibility and multiple flavors of markdown.
But if people implementing their own markdown processors try to write a spec or grammar for what they are implementing... he is not fine with it, and considers legal action? It's okay to write a markdown parser, as long as you don't write a spec for it?
There may be a way to justify that position rationally, but I doubt there is legally. He's already abandoned any claims to trademark, by making it clear he's okay with people making their own markdown parsers and describing them as 'markdown parsers', without his permission.