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. 

FOSS doesn't mean necessarily mean decentralized.