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TDP numbers from Intel don't tend to mean much. They group a wide range of CPUs under the same TDP.

The CPUs will generally use far less power than the TDP might suggest.

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.

No integrated GPU.

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.