Dude created a simple tool based on email conventions of the time and released it to the world for free and it's still with us, used all over. And you're here crying that he didn't want to work with you? Did you think he was somehow obligated to? How incredibly entitled.
It's open source, so you do what you do. That's the good and bad of it. No one owes you a god damned thing.
I think you're missing much of the context and are misrepresenting what happened.
As far as I'm aware, there was no crying he didn't want to work with people, but there was a frustration that he was not open to having a Markdown "standard". To the point where he actively opposed efforts to standardize it, at least under the name Markdown[1].
This is legally and technically fine, as he owns a trademark for Markdown, but when you combine the inconsistent application of that trademark (GitHub Flavored Markdown is seemingly fine, but Common Markdown was not), along with him calling it "Jeff Atwood's crusade" and mocking the effort[2], it's not a great look and resulted in quite a few frustrated people.
As an open source project, you're right that he doesn't owe anything to anyone, but that doesn't mean people have to be entirely happy about how the situation was handled either.
But it’s not an open-source project. It’s a download link to a Perl script which is never updated and effectively no one uses.
There’s currently zero value to the code he wrote or the reference in his page. The only thing of worth that remains of his original implementation is the concept and the most basic syntax.
The point is Gruber was contacted out of courtesy, his name as the initial author carrying some weight in people's minds, and he reacted in an openly hostile and mocking way.
It's open source, so you do what you do. That's the good and bad of it. No one owes you a god damned thing.