Gruber also mentioned it recently on one of his talk show episodes. I cant remember which episode, but as I recall he was not a fan of the effort at all and thinks it is not necessary.
But I suspect it's also in part simply that Atwood and company never said in public "we are doing this." They just announced it as a fait accompli. Yes, two years ago Atwood and Gruber tweeted at one another briefly and Gruber said he wasn't interested in formalizing the spec. But that's not really an attempt at having a discussion, and it sure as hell isn't an announcement of a project.
I think a formalized spec for Markdown is a good thing. But the way this was handled was still kind of a dick move, and I don't think it had to have been.