That's not true tho. f-droid supports (true) https://f-droid.org/en/docs/Reproducible_Builds/ for quite some time now. Those are signed by both, f-droid and the author.
https://community.signalusers.org/t/signal-android-app-on-f-...
F-Droid with reproducible builds signed by both parties seems the best of both worlds to me, now I don't understand why Signal is so stubborn about this.