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That’s just open source, not decentralized. I can’t find a definition of decentralization that would even make it vague. From Wikipedia:
Signal has a central authoritative server and to use it with any other server you have to modify the source code.
Imo this fits my usage of the term — Signal can be broken up into many isolated servers [1] all offering the same service.
References
Depending on exactly how said open source development is occuring, I could argue that open source development is an example of decentralization. It may even be an example of federation (all depending on licensing and development medium imo).