But then these “most implementations” are not implementations of “standard Markdown,”
Note the qualifier—“most implementations that work with client data”. Markdown is also used extensively to generate static content that is not user-generated.
There are still multiple implementations used in generating static content, no two of which really do the same thing, e.g., pandoc, multimarkdown, and etc. These are all still arguably “markdown” (at least, they call themselves that) but don’t conform to standard markdown as you understand it.
Note the qualifier—“most implementations that work with client data”. Markdown is also used extensively to generate static content that is not user-generated.
There are still multiple implementations used in generating static content, no two of which really do the same thing, e.g., pandoc, multimarkdown, and etc. These are all still arguably “markdown” (at least, they call themselves that) but don’t conform to standard markdown as you understand it.