There's "decentralized" in the sense that every device runs the whole stack. In an analogy to another protocol, this would be like running SMTP and IMAP on your phone and laptop.

Then there's "decentralized" in the sense that the protocols that govern are open and anyone can plug in without permission. This is how email works in practice. Most people do not choose to run their own email servers, but they nonetheless benefit from the fact that people who are interested can do so and provide email service.

Bluesky is the second kind of decentralized.