Basically it said to be a true decentralized system people have to be spread equally through the system so the power distribution is equal and not concentrated on one part of the system.
So mastodon is not truly decentralized because a lot of the users are on 3 instances. Also that’s where the fallacy lies - because people choose to be part of the larger instances
As long as the technology is not accessible to everyone from old folks to 5 year olds, the power is concentrated to those who understand and maintain it.