Think of it as a general rule of thumb, a statistical average. You will for example find that often managers and executives make a crazy amount of money, but are all but useless, but are only a small part of the bigger company.
Should each owner run around and tell each employee globally how to do their job, what is the goal, budget, restrictions, rules, timelines etc? This is a massive vacuum that needs to be filled, and managers fill it. Some better, some worse, just like any other profession.
Generally weird article to be polite, probably put together in a haste to give benefit of a doubt to the author.
One problem is that the amount of "vacuum" that needs to be filled is often also decided by the managers, and many of them start creating work for themselves. Suddenly every team and every project needs to have a separate Jira and Confluence instance, there are meetings every day but most of the meeting is just reviewing the same information you have reviewed yesterday, etc. And the budgets, yeah, with so many managers there is hardly any money left to hire anyone else.
Of course not, but there are many useful ways to organise this. Hell you can still have managers, but make them elected from the body of workers. It's more that because of the first thing you said (with which I agree) these managers are basically cops or wardens.
They are all but useless, sure, but much, much more valuable people as they hold a scarce property that nobody else working for the company has: Trust. Value is defined by the relative scarcity of something desired.