Performance per TDP is always relevant. Sure individuals may choose they just want whatever to maximize a metric, but that’s a different decision entirely. Like comparing a motorcycle and a truck without mentioning all the things different between the two.

TDP is the wrong measure here*. i9-13900K and i7-13700K both have 125W TDP with 253W PL2 specs. By that calculation, the 13900K is more performant per TDP, so is "better" for efficiency.

Computation per kWh (or rate of computation per kW) is the right efficiency metric, not TDP (thermal design power).

* - or at a minimum isn't precisely the right measure here

If you have a workload that pushes the 13700K and 13900K to their turbo limits (and have ensured that those limits are actually configured to be the same), then you really will find the 13900K getting more work done for the same power and energy. That's how the extra cores help in power-limited scenarios.