It used established syntax, but it didn't start there. There are also other markup-languages, which use other established syntax from the old times. A language is not just some random parts.
> You could print out a Markdown document today and jump in a time machine to 1970 and hand it to someone and they'd understand it.
Because it's human-friendly by design, not because it's syntax already existed back then. There are limits to what people would understand in 1970 from the syntax.