I think that is actually the entire center seat back folded down and we are looking at the back of the headrest.. which might have cup holders.
We know the seats fold down for storage, and the location and size of that element (center of center seat, all the way forward on the bench) is completely wrong for armrests. The armrests (as shown in other photo by Markb1) are where they should be (between seats) and are ~2" x ~16" which would never reach the front of the bench.
So I think that is the headrest, though the height of the seat back would normally fold further forward than that (I assume) with the headrest forward of the front of the bench. Perhaps it hinges forward, then slides backward to fit flush to the floor of the trunk, while also leaving cup holders in a perfect position.