They are using repeatable builds, so the fdroid version is signed by fossify. You can update between github, izzy, and fdroid, and in the future likely also google play, if they release there.
I dug up the meta discussion on it. Mentioned there is the issue to add reproducible builds to the phone app, which is marked complete since that was released on github and izzy last week.
Further for me the app is marked installed on both entries in fdroid, which means the fdroid and izzy repos should serve bit by bit identical apks, from my understanding (the installed mark being based off of the apk hash).