Every writer has to market their work these days. Even people who are published with big publishers become marketers. When I published with Chicken Soup for the Soul, they suggested all kinds of ways to market the books. So I don't see anything wrong with it, plus if you think Medium has value, it's a way to promote value to the reader. All it takes is copying the link at the bottom of the article, so it isn't much trouble. I don't do it all the time, but I've had four or five people to sign up through it.