Yes, it's centralised (to a degree), but when people talk about decentralised protocols, they usually mean that the protocol itself is decentralised. There's still usually an official or de-facto standard maintained by a relatively small group of people.

Your argument that Bitcoin is not decentralised could easily be applied to any P2P protocol. I don't think there's any P2P system around, except perhaps natural language, which has a decentralised specification.