Here's the relevant quote:

The F-Droid project cannot require that developers register their apps through Google, but at the same time, we cannot “take over” the application identifiers for the open-source apps we distribute, as that would effectively seize exclusive distribution rights to those applications.

I think that last sentence is saying that it would work, if developers decided to exclusively distribute to F-Droid and effectively gave up control over the app to the F-Droid team.

I'm thinking there might be a possibility to register the same app under two different identifiers, one controlled by F-Droid, the other by developer.
But yeah, this makes some things more complex and might be deemed malicious behaviour by Google.