Back in the early days of aero they had solid frames, i.e. the whole triangle was filled. The surface drag was so high that it made the frame slower than a normal aero frame.

This is the idea with the Kamm design, where if you have the first 2/3 of the aero shape, you can skip the last 1/3. Air will flow as if that last 1/3 is there - look at a Prius - it has a dramatic dropoff in the back (no long tail) but it has a very low Cd.