I didn't know about these, I remember they were against any attempt to make an open-source alternative client in the past, but either I was confused or they changed, so that's good news, at least they're not lying about the encryption.
Not quite accurate. Signal for Android has reproducible build and there is also Molly, an alternative Signal client.
https://github.com/signalapp/Signal-Android/blob/main/reproducible-builds/README.md
Discussion
Their encryption is top notch tbh. MLS is probably better but they're doing some crazy shit to avoid knowing things like group memberships, group roles, etc.
It's quite a lot but I'll drop this here for anyone who's crazy enough to read it https://eprint.iacr.org/2019/1416