f-droid represents a distro model with a trust model and certain requirements including build ones. Think of it as the Debian of Android.
When f-droid support is mentioned, it's not a question of mere APK availability. All APKs after all are available if you know where to look. And it just happens that f-droid compatible 3rd party repositories already exist without the strict requirements (e.g. IzzyOnDroid).
Beyond warring against any kind of third party build (don't let me get started on forks), Signal uses google services dependencies, so it's not just about the source of the binary APK. Note that whether these dependencies are hard, or "just!" a default, is not that relevant in my books (this is an active point of contention across many many apps).
This insistent attempt at defending this blessed supposedly secure messenger seems infinitely weirder than what any fanperson of any other app can muster! I put imaginary hypotheticals that don't even understand the point of f-droid, like "it is perfectly capable of enshittification" into that bracket. The continuous attempt at painting a false binary of Signal and Telegram is even more pathetic.