How exactly do you set up a system to be centralised, then "leave it to be decentralised"? That's a centralised system.

For example, take a factory machine: you set it up to make things, it does it on its own. When it fails, it can't fix itself, so you need something else to step in and fix it. That's a centralised system.

A fully decentralised system would be a factory machine that never needs anyone: it works on its own, fixes itself, etc. That's what it means.