You saw that I expressed doubts about whether Gruber’s Markdown documentation also is or isn’t falling under the license in question? This documentation undoubtedly is used by everyone in the Markdown universe to derive their own implementation flavors from.
If the question is “can derivatives of a language, which is based on several decades of observable idioms in email and chat conversations, fall under the conditions of a license, simply because the subject of that license happens to contain some documentation loosely describing said language” – I would say absolutely not.