Hello everyone!! This is my first post, been long time reading and trying to learn from all of you a bit, and now have decided to join this great site
For me that is only a shadow from the rear wing endplate, you can see the shape of the 'darker area' is exacly like the lower place of the endplate, including the slit. You can follow the shadow from the endplate and see it has the same shape, and it even goes up to the beam wing.
And those edges are from the connection of the diffuser to the crash structure. But they are flat against the crash structure, no '3D' there.
I agree that, all combined, it seems a bit like a deck, but watching it closely (at least for me) it is not.
Regards!! (and please excuse my poor English)