Blockchains are decentralized in that anyone can participate in the network. How those voluntary participants interact with one another once they join a project (a specific blockchain protocol) is a problem of governance. You can think of this discussion as "How to develop a decentralized governance structure." What the author is saying is that anywhere you have people interacting with one another, governance structures will emerge. You can either consciously design them, or not.

IMO this article is addressing a larger point of "What the heck does decentralization mean?" It's often easy to fall into the trap of believing that "decentralization is always better" as if decentralization is an easy thing to pin down. The United States, for example, is decentralized in that it has individual States that have some level of autonomy. But it's obviously also highly centralized in that it has a Federal Government with a lot of compulsory power. Point being, centralization and decentralization are often in the eye of the beholder. They are more subjective terms than people often let on. The truth is that the challenge is having the right amount of decentralization/centralization. The reason it is so important that these systems be entirely voluntary is that this serves as a check on centralization (among other things). If the participants feel the system is becoming "too centralized" they can always leave, so it's always in the best interests of those involved to keep their system "only as centralized as necessary."