Yeah. Basically it builds and then sign the app with their own keys, not the developer's. The problem people has with this approach is that if F-Droid suffers an hacking attempt, the attackers could mess with the apps.

The team behind F-Droid is already trying to fix that with reproducible builds. It means that an APK downloaded through F-Droid could be compared to a GitHub release, for example, and they would have the same key.