MarsEdit developer Daniel Jalkut on Medium’s new API, and its support for Markdown:
Unfortunately, the Markdown support through the API seems to be a
one-time conversion from Markdown to HTML, at the time of
submission. This means users can write in Markdown for the initial
composition of a post, but any further edits (through the web
interface only, see above) will need to be done using Medium’s
default rich WYSIWYG editor.
A more attractive long-term solution for Markdown fans would be to
support storing Markdown text literally in Medium’s database, and
converting it to HTML only for presentation on the web. This
leaves the pristine Markdown available for perpetual edits either
moments or years after the post is first published. This would
require updating the web interface to support editing content as
plain text, but would be a welcome change for anybody who favors
editing in Markdown.