The irony of saying "don't be a serf! build your own products!" whilst depending on the Medium platform is either delicious, or painful. Not sure which.
Just like your codebase is bound to have dependencies, so will your company. If he's not depending on Medium, he would be depending on Google for ranking, hosting company for hosting his blog, Mailchimp for sending out email marketing campaigns and so on. The difference is doing the work according your own decisions versus dealing with decisions made by other people higher up in the company according to some agendas that may not align with your needs.
If that equivalence were true, there wouldn't be any irony to mention. Technical elements are replaceable parts, unless some abysmally bad decisions have been made. Hopefully you/your firm does not need Mailchimp's continuing permission to exist. Any of the many competing similar providers will do.
In contrast the company in question has the tagline, "Make more money on Medium". Their existence is predicated on an external indulgence, and those are foundations of sand.