It means OWS has knocked down yet another blocker in the path to inclusion in F-Droid, namely a widely available GCM-free APK build that we can ship in F-Droid directly. Now we need to "make sure F-Droid can build Signal reproducibly" and then we can just using the resulting build and compare it with the build provided on signal.org (or, more accurately,
updates.signal.org e.g. Signal-website-release-3.31.4.apk).
Then if we don't want to piss off OWS, we would need to have crash reporting, stats and app scanning and we're pretty much done. The trickiest issue is probably crash reporting, which seems to be tightly coupled with the play store still - we'd need something like ACRA to be pulled into Signal for this to work, presumably.