"AMD's Zen 5c architecture...density-optimized cores that are conceptually similar to Intel's e-cores"
No, they are not. Intel castrated their cores to make them smaller. AMD packed the transistors closer together and reduced cache size. From instruction stand point, Zen 5 and Zen 5c are identical, which is not the case with Intel's e-cores (Also refereed to as Garbage Cores)
Castration takes something away that was originally there. Not the case with e-cores, they are a distinct design for the same ISA, sometimes with a subset (still no castration).
Yes, turning off the AVX512 IP blocks on Alder Lake P-cores probably qualifies as castration.
And for the AMD compact core approach, a half sized cache might actually fit your use of "castration" and result in significant penalties for cache sensitve workloads. That's why hyperscalers will carefully test and tune.
But the genius is that a lot of the silicon surface area which has to be left dark for cooling on top-clock desgns, can instead be filled with logic on a design with lower max-clocks, resulting in less area waste.
Or well, it would be genius, if it wasn't quite simply what Ampere and similar hyperscaler specific designs had already been doing first.
If now they could only make chips switch between those two personalities on the fly!
And I guess dynamically disabling/not using hyperhreading to gain some clocks would get you half way there?