Isn't it a bit early to have thoughts about something we don't know the UI/UX of? Could be that "Markdown support" is just "Import/Export as Markdown", or even just export. Or it could be a fully fledged WYSIWYG editor.
The rumors seem to indicate just "Export as Markdown", which seems to be exactly what Gruber wants, according to the last 10% of the blogpost. So the rest is ranting against an implementation that doesn't seem like it'll happen?
Notes is already a WYSIWYG editor, with a feature-set exceeding that of plain Markdown (handwritten notes, math formulas and plots, colored highlighting, etc.). In the general case, Markdown export and re-import would likely be lossy, or would have to use HTML elements for non-Markdown features. The main question IMO is if they’ll add Markdown source visualization and source editing, in addition to export/import. It could conceivably even just be export, without import.
Gruber is famously protective of the original Markdown specification and his specific use case. He’s reacting to others’ expectations and clarifying his position that a “Markdown editor” is a bit of an oxymoron. He supports that position by reiterating his inspiration for creating the format.
I understand his point, but I disagree with it. Markdown wasn’t invented for the purposes we use it for today, true. And yet the most popular programming editor today is a website running inside a modified browser, themed with CSS and extended with JavaScript.
We have a tendency, as a group, to push things beyond their original intent.