@Moem
Well I'm not sure, but it seems like the signalbot server : tchncs.de is the one who connects to signal, and fetches messages, decrypts them, and then redirects them through matrix. So they get to have the possibility to see unencrypted messages, which voids a bit the utility of signal.
One interesting thing would be to be able to host the matrix bridge at home, or even on the phone itself, but seems a bit complicated.