“No Google Play services” falls under “app must be FOSS”. The average publicly developed open source app should not have much trouble getting into F-Droid if the developer wants to. Google Play services consists of several components, one of which is a proprietary library included in apps using it. If your app includes proprietary code, it is not FOSS.

If Signal decided a build without proprietary blobs isn’t worth it, they’re not getting into F-Droid. Forks of Signal exist that remove the Google Play services build requirement, those are in F-Droid.