A hypothetical scenario where software scheduling has no issues doesn't change the testing done in reality, where the 8 core 7800X3D is always faster than the 6+6 core 7900X3D. Frequency isn't the only difference.
When scheduling works, and there's no issues with latency, you're right, the 7950X3D is faster than the 7800X3D, the problem is it doesn't always work in reality, as testing shows.
You can undervolt non X3D chips to get the same efficiency gains.
The X3D chips can undervolt further because they don't have to hit the same frequency targets. You can't expect a 5 GHz chip to undervolt to the same voltage as a 4.5 GHz chip, while maintaining performance.
Caveat being disabling the second CCD on a 7950X3D, it would always be faster than a 7800X3D if thats what someone is really after.
Not to mention with proper thread assignment there are games that primarily run with a main thread or dont fully utilize all cores/threads on the 7900X3D/7950X3D that are faster already.