In a literal sense, of course posting on Medium is self-publishing. I was using the term to refer to the production and (ideally) sale of ebooks, possibly in conjunction with print-on-demand. While Medium’s business model isn’t transparent, I’m inclined to give them the benefit of the doubt and assume that they’re not engaged in some Amazon-style long-tail exploitation. At the very least, posting on Medium doesn’t cost me anything in terms of cover design, marketing campaigns and the like.

I don’t think I have any reliable answers except that the immediate imperative seems to be to find readers somehow. And the ways of finding readers that are typically recommended to self-published authors all seem at best to be several years out of date.