My guess is the author meant “owns” in the informal sense of “can broadly do whatever they want with it,” not in the formal sense of “possesses an exclusive license to it.” You retain the license to content you post to Medium, while granting them specific non-exclusive usage rights.

That said, if Medium represents the only copy of your work, it should be considered at-risk. They are not obligated to retain or make it available in perpetuity. I agree with the author that this is not a desirable situation, and that you should have full control over the source of truth for your intellectual property.