It just means "the description written by Gruber at his domain." The Perl script is just a convenience (if one can call Perl convenient...) for the curious.

Anyone else's flavor of Markdown can be used by specifying its identifier, e.g. "com.github.markdown.v3" (I made that up, it's not a real thing AFAIK)

It seems fine to me. If your app is using weird edge cases of markdown syntax, or extensions, it should probably specify a different UTI. If you’re pretty much just doing the level of markdown in the average rich-notes app, that’s pretty interoperable.