Coinjoin is a mixing method. And just as any mixing method (besides exceptions), it can be implemented in both centralized and decentralized manners. Central manner is the Wasabi wallet, where the coinjoin users must gather around a server, which will do the coinjoin without infringing custody. There's an even worse way, that is to just give up custody on a mixer, and let it do the coinjoin. The best way is to use peer-to-peer software such as JoinMarket, which works decentrally.