Do you know the term TiVoization? Wikipedia article: https://en.wikipedia.org/wiki/Tivoization

Signal is opensource, but you can't use your fork in the official server. You can use it on your forked server, but then you can't communicate with anyone that's not on your server. So being opensource is just a marketing tool. There's also no way of knowing if the binaries distributed were produced with the source code they published. There's no advantage whatsoever to be opensource for the user. So "opensource" in this case is just a marketing term, like the in case of TiVo.

The Signal app is open source but forking it and connecting it to Signal servers (signal it's centralised) is againt their TOS