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.