What is the utility of such a definition? As far as I am concerned, anything I can read with my editor is plain text. That definition is trivially useful on a daily basis. I don't see any point in calling markdown something other than plain text. Because it's just plain text.
And of course, I intend deep disrespect that you had the gall to claim correctness for such obviously arbitrary definitions.
If I can open your file in a notepad and effectively edit it without the format changing or getting corrupted at saving its plain text.
Not plain text: Attempting to edit an .exe file in notepad.
Unfriendly plain text: Minimized javascript where the entire file exists on one line and the human readable elements jumbled together.
Plain text: Your average source code file/html file that attempts to adhere to something around 80-120 columns of text.
Personally, I find 'human readable' to be a better term for your definition and use 'plaintext' to mean either unformatted text (except perhaps with whitespace), or the non-markup text within a marked up document.
Wiktionary suggests that the divide is contextual, with your definition being the 'file format' definition and GP's definition being the 'computing' definition.[0]
A .txt file for notes is plaintext, because the language I'm using doesn't have to be compiled for my goal. Programming languages are not, because the expressed language is compiled into some other target language (machine code).
Markdown is not, because it's compiled into HTML.
A .txt undergoes no transformations from my writing, to its storage, to my later usage of it.