That is not what was being discussed, here.
"you should only release what you're willing to maintain"
The Markdown story was not about that. Nobody demanded John's involvement; all people wanted was for him to let the project go. That's a far cry from "willing to maintain."
Why should he have to do even that? It's his project to do with what he pleases. Work on it, horde it, release it under the AGPL... it's his, nobody else's. Of course, that's not their impression. Their impression, and the general expectation, is that if someone open sources code, they do so expressly for the good of the community of developers.
You see this quite a bit in GPL discussions as well.