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I don't think availability via any particular distribution mechanism is a reasonable litmus test for "open sourcedness." Such a test would have excluded Linux for most of its development history, prior to migration to git.

This particular distribution (F-Droid) requires that the source be build-able from strictly source files, without bundling binaries.

You can download the APK directly from Signal: https://signal.org/android/apk/

When a new version is released, the Signal app notifies you.

Problem is, the APK is a closed-source binary. What they include there might be completely different from what is in the source.

Not with reproducible builds, no.

One more point in favor of Matrix/Element.