Yes, its all part of the integrity of the transom. The hull vents have pieces that are glassed to the inner hull and the transom, also the corner boxes add a bunch of structure to the corners, even though they are just screwed on. In this picture you can see the 'scoop' that I have glassed to the inner hull, then I smoothed the joints out with putty.
You can see the white filled seams in this one.
I popped the whole cap on and laid one layer of glass, to join it in the corners I was amazed how much it kept the 'twist' down while walking around inside it.
Underside of the cap.
Ground the seams and installed with screws to adjust it
I can say from trying to moving a boat around with no structure, they flex a TON without anything in them. It was very touchy to get it squared back up once it was in the shop on the concrete floor with just a transom core in it.