From what I can see, they are using the whole rectangular are between the lower part of the rear wing and the 'lower diffuser' as the third deck. Effectively, everything under the 'bridgestone' logos is a diffuser. That is a whole lot more than the others, which seem to have a rounded shape around the crash structure.
I wonder if that is where they are saying that they explored the rules more than others, or a question of being able to feed this whole thing.