Is Fennec F-Droid an official product by Mozilla? Is the Fennec F-Droid Browser an official product by Mozilla? All the links from the F-Droid page link to official Mozilla pages, but I cannot find any hint on one of the Mozilla pages that they distribute Firefox under the name of Fennec on F-Droid.
I agree that the description is ambiguous on this matter — i.e., is the Fennec F-Droid app an official Firefox release, or a fork like Orfox or Iceweasel? The different branding implies the latter, but the lack of any dedicated repository and all the Mozilla links imply the former. The answer is somewhere in between.

The first thing you have to understand is that F-Droid does not package any supplied apks, but instead builds everything from source. Therefore, nothing is truly an official release from another project on the main F-Droid repo, even if that project gives F-Droid its blessing. At the end of the day, you're trusting F-Droid's build environment, and not that project's. The way a project is added to F-Droid's app repositories is by adding a metadata file to a particular git repository that specifies how to build it. In the case of Fennec F-Droid, that file is here:

https://gitlab.com/fdroid/fdroiddata/blob/master/metadata/org.mozilla.fennec_fdroid.txt

While that looks like a 4,354 line, single-file build script, and yes technically it is, the overwhelming majority of it is just repeating the same  ~40 lines for every release they've shipped. Those lines are basically just passing options into the build process (e.g., use the generic branding, build for ARM CPUs, etc.), and don't contain and code that needs to be compiled. All of the actual code comes from upstream.

It's also worth noting that this isn't some random person's hobby project, the way a lot of F-Droid apps are. The main contributor to that file is relan, who is currently the largest contributor to F-Droid's build scripts, and a prolific contributor to F-Droid in general. Mozilla is also aware of Fennec F-Droid, and does what it can to help. For example, see this thread:

https://forum.f-droid.org/t/making-it-easier-for-f-droid-to-package-mozilla-firefox/1649

tl;dr: It's as close to an official release as you can get without being one.