Yes, the app is FOSS, but the app is the least important part of F-droid. The key is the catalog of apps that is extremely controlled by a few individuals that review. To give you an idea, Firefox is not opensource enough for F-Droid's review board. Which is bonkers.
To me, as an app developer that publishes constantly to both Google and F-Droid, the F-droid review board has been way more controlling of what I can do in the app than what Google does. Which is also bonkers.
But again, neither Google, nor F-droid is there to decentralize things from themselves. Both are incentivized to keep as much control as possible. And that's my hole point to move away to Obtainium.