> Since md backend is derived from html, is it necessary to define an 
> option specific to markdown or the value defined for HTML may be reused? 
> I am unsure which variant will be more convenient, so it is not more 
> than an idea that may be easily discarded.

I considered reusing the value from the HTML exporter, but I rejected it,
because the org-html-toplevel-hlevel defaults to 2. Reusing it would alter the
existing default behavior for markdown exports, which I thought was undesirable.

If you think it's okay to alter the existing behavior of Markdown exports, I
could change the code to use org-html-toplevel-hlevel to control the top level
heading level of both HTML and markdown exports.

Thanks,
Rohit