Their problem is that F-Droid releases are signed by F-Droid, not by Signal. This way F-Droid could potentially insert a backdoor in an update.
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.
Google requires app developers on play store to give goole the keys that enable google to insert backdoors in any release. I can't trust anything on the play store for this reason. There is no way to tell which apps have been backdoored by google for whatever reason (the usual reason is a NSL).