To be clear, Signal is not available from F-Droid. The above link is about a fourth party publishing a Signal build in an f-droid-compatible repository.

f-droid ships libre software only.

3rd party repositories ship whatever.

No, and I don't want to rely on f-droid for anything important due to their shoddy security practices (+ as a sibling comment says there's no official signal binaries on fdroid)

For apps i do install from f-droid repos (official or otherwise) i prefer https://github.com/Droid-ify/client