Why would it be much cheaper? The chips are intentionally clocked higher than the most efficient point because the point of the CPU is raw speed, not power consumption, especially since 7950x is a desktop chip.
His point is that M-CPUs being somewhat competitive but much more efficient is not as stunning since you're comparing a CPU tuned to be at the most efficient speed to a CPU tuned to be at the highest speed.
Similarly a 4090's power consumption drops dramatically if you underclock or undervolt even slightly, but what's the point? You're almost definitely buying a 4090 for its raw speed.
AMD's chiplet-based design means they have plenty of other ways to make good use of parts that cannot hit the highest clock speeds. They have very little reason to do a 16-core low-clock part for their consumer desktop platform.
And your concerns about "a software limit that may or may not work" are completely at odds with how their power management works.
It isn't a "software limit" beyond just being controlled by writing to certain CPU registers via software. It's very much a feature of the hardware, the same feature that allows for overclocking the chips.
That's not how binning works. Quality silicon is also the ones that are more efficient. A flagship 65W part would be just as expensive as a result, it's the same-ish quality of parts.
His point is that M-CPUs being somewhat competitive but much more efficient is not as stunning since you're comparing a CPU tuned to be at the most efficient speed to a CPU tuned to be at the highest speed.
Similarly a 4090's power consumption drops dramatically if you underclock or undervolt even slightly, but what's the point? You're almost definitely buying a 4090 for its raw speed.