This account is a very selective description of the plaintext landscape of the time. Ignores asciidoc and org-mode, both of which are in their different ways more suitable for some of the current things markdown is expected to do.
Understanding Gruber’s precise work (tech blogger, journalist) explains some of the peculiarities of markdown. And why markdown isn’t great for those of us who write for print formats.
It’s a pity that markdown has ossified into different variants and immutable ‘standards’. Also a pity that the format is text only - it would be easy enough to extend to save images rather than only link them; links nearly always break in the long-term.