Decentralized systems can be conceptualized along three axes, forming the Governance Cube.
If you divide the system in half will they continue to function or fail catastrophically.
Can you combine systems which previously ran separately into a larger one, without losing any functionality or having clashing indexes/hashes.
Is the system controlled by a single individual or a single organization, or is it controlled by many people or different organizations.
Do you need anyone's permission to run the system.
How many computers is the system running on, and how geographically dispersed are they. Are they all in a single warehouse or in many homes and offices.
Bitcoin: Logically centralized, Architecturally decentralized (apart from mining), politically decentralized (mostly)
Traditional corporations: Logically centralized, Architecturally centralized, Politically centralized
Fediverse: Logically decentralized, Politically decentralized, Architecturally decentralized. Fediverse might be considered politically centralized, but only at the instance level.
Signal App: Logically centralized, Politically centralized, Architecturally centralized (single server)
SSB: Logically decentralized, Politically decentralized, Architecturally decentralized
A system which was politically centralized but logically and architecturally decentralized would be where everyone could run their own solid-like data pods and control permissions to their data, but identity would be centrally managed by a single organization.