Out of the loop when it comes to the latest processor technology. How does this chip maintain the same TDP as a 6850K but with 4 more core? Same size (lithography), roughly the same freq, memory size/types.
Power/frequency scaling is quadratic since voltage can decrease with frequency, so even the 10% difference in base clocks could explain it, depending on where in the voltage/frequency curve they are.
Plus, this is Intel's 3rd iteration on the same process; even if the feature size doesn't change you can extract a bit more power efficiency with 2 years of feedback.
Er, neither has a GPU. The i9-7900X has the benefit of generation of tuning and runs at a slower base clock (3.3 vs 3.6 GHz for the 6850K). Slightly smaller l3, and a much larger but also higher latency L2 helps as well.
I suspect moving from a ring bus to a mesh helps as well.
The CPUs will generally use far less power than the TDP might suggest.