I’m a user of Ulysses too. As far as I can tell, their integrations are internal and there’s no API, i.e. they’d need to collaborate on building a Discourse integration.
It looks like they currently use Circle as their community solution. Perhaps someone could pitch them to migrate to a community solution (Discourse) that reflects their stack and ethos (e.g. markdown), and collaborate with them on an integration at the same time.