It's just CommonMark, Gruber was ticked off enough that he declined to allow them to use the term Markdown at all. Alone among the variations, or nearly so, he's fine (as your link indicates) with Git-Flavored Markdown.

The thing is, they didn't fork it, they decided to "standardize" it. John Gruber had already published a Markdown standard: https://daringfireball.net/projects/markdown/, and a reference implementation. He's fine with variations on his standard existing, even somewhat encouraging, but took it as a personal insult that others set out on their own initiative to "standardize" something which already had both a standard and an implementation of it. I don't blame him at all.

The outcome isn't so bad, really. We have a whole family of variations on Markdown, which is sometimes annoying for inter-op reasons but it's a lightweight plain-text standard, making a few tweaks to parse it with a different engine isn't going to ruin your day. CommonMark ended up being an acceptable standard for a minimum-but-extended dialect which the diverse implementations can (mostly) implement, which was the goal, and better yet, people don't get to be dicks about "that isn't Markdown" by reference to the CommonMark standard. Gruber made sure of that, and bless him for it.