That's interesting, thanks for drawing my attention to it. I would need to go spelunking around to see how they reference an .apk from the Play Store, which I got the impression used crazypants gRPC shenanigans for building download URLs
This makes it a bit trickier, yeah, though if the developer can get an APK signed with their Play App Signing key, and the app in question is a reproducible build, they can then publish it in F-Droid: https://fdroid.gitlab.io/jekyll-fdroid/docs/Reproducible_Bui...
(and probably they can upload it to their GitHub releases or something so that F-Droid picks it up from there)