But you do now know about it. People telling each other about a hidden feature (via HN or whatever) is a common and valid way for non-essential features to be discovered.

If you designed UIs assuming that that sort of user-to-user education doesn't exist, you'd be forced occasionally to reject a potentially handy power-user feature on the basis that there's no good place to promote it without cluttering the design. Which would be a shame.

Discoverability isn't the be-all/end-all in User Interfaces.

It's just a shortcut, not essential. You have help links in each Markdown-capable textbox anyway.