It seems to not be needed or used. My take from the thread is that F-Droid complains about the library reference present in the source code at all. It does not know the library is not linked in the F-Droid build.
AFAIU if a nonfree library can’t be disabled or patched out, the app doesn’t get to be published on F-Droid at all. The antifeature description[1] just says
> In our experience, where the upstream developer includes Non-Free libraries, sooner or later they will include more Non-Free libraries, or other Anti-Features. Frequently they become impossible to maintain/update in F-Droid.
> In our experience, where the upstream developer includes Non-Free libraries, sooner or later they will include more Non-Free libraries, or other Anti-Features. Frequently they become impossible to maintain/update in F-Droid.
[1] https://f-droid.org/docs/Anti-Features/#UpstreamNonFree