You can already do quite a lot with this awesome project: https://github.com/AsamK/signal-cli. It doesn't support everything though.
From what I understood from the LibreSignal discussion: Open Whisper Systems provides the source for transparency only, not to enable others to make clients for their service. They're free to build their own servers (which obviously will be unable to communicate with official Signal users!) but not connect to Signal itself.
The reason I keep pointing this out is that I'd really love for them to change their stance on this or at least allow federation from third-party Signal servers. It would really make it an open network. I don't want to move to something that is as locked-in as whatsapp as it is similarly dependent on their choices.
Does he? I doubt he really cares if some devs build a few projects around these kind of tools e.g. hobby projects.
Either way, this project has existed for quite some time and the author has maintained it for quite some time now. It seems to be going strong.
He's very much opposed to other projects using either the name Signal or the servers they provide.
LibreSignal abandoned their project because of his comment. No legal action was taken and I think this is why libsignal-client is still running. Open Whisper Systems never bothered to sue them, probably because it's only a fringe phenomenon right now.
His opinion is clear though and it leaves it open to legal action in the future.
I agree it is his perogative but I wish there was an actual open network. I don't like moving from one vendor-lock-in (whatsapp) to another (signal). I want something where I can do with my conversations what I want, like Matrix. For this reason I don't advocate Signal to my friends as an alternative for Whatsapp.