I like the wiki definition

https://en.wikipedia.org/wiki/Decentralization
Decentralization or decentralisation is the process by which the activities of an organization, particularly those related to planning and decision-making, are distributed or delegated away from a central, authoritative location or group and given to smaller factions within it.

Based on this and other definitions I've seen, Bluesky is NOT decentralised.

I struggle to see how a platform of which 99.96% of it's users are controlled by one entity is Decentralised.