They're talking about the space in the middle. If you go to the top of the second page of the teardown ("step 10"), they're talking about removing the heat sink that's attached to the logic board. That sort of thing is surprisingly absent from other consumer gear.
I had to re-read the teardown before I became unconfused. You are correct, and it
is nice to see attention paid to proper cooling.
I will say that my 5th (just prior to the new design under discussion) generation Airport Extreme and Time Capsules do run warm, but have never given me any trouble. They are sitting in open air with plenty of air circulation.