it’s decentralized

No it’s not. From literally your own comment:

Signal relies on centralized servers

I was using "decentralized" to mean that there isn't centralized control over ownership of the service in general — eg anyone can spin up their own server (impractical, imo, pushing it more towards being centralized) and people can use it (making it decentralized, imo (Please correct me if I am wrong, but I do think my usage of the term is appropriate in this way.)), but people who use that server can only communicate with that server (making it not federated). But yes it could still be said to be centralized in that it operates on a client-server model ^[1]^.

This is more an argument of definitions, though. I'm not trying to claim anything in bad faith.

That’s just open source, not decentralized. I can’t find a definition of decentralization that would even make it vague. From Wikipedia:

Decentralization 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.

Signal has a central authoritative server and to use it with any other server you have to modify the source code.