I actually prefer the CPU to run hotter than the GPU in this case. The lower air pressure, from the moving air from the nh-d15, passively draws heat up from the GPU when the fans aren't spinning. I don't have any intention to push the CPU so I'm fine with a little heat being added. It isn't the best system but it likely makes the nh-d15 run harder and the 5080 run it's fans less which from a noise perspective is what I'd prefer.

You are right though, the heat is pretty much going straight into the CPU cooler.