My guess would be that this is because of thread migration.
(After reading the TFA: that's what Agner says right there in the 4th paragraph.)
(After re-reading the comment: I guess that the OS changes would need to be extensive with little to no benefit: running AVX2 on all cores will likely be faster than running 2 P cores with AVX512. The only thing that is really affected is the code that could use AVX512_FP16, but I doubt there's a lot of it outside of Intel.)
(After reading the TFA: that's what Agner says right there in the 4th paragraph.)
(After re-reading the comment: I guess that the OS changes would need to be extensive with little to no benefit: running AVX2 on all cores will likely be faster than running 2 P cores with AVX512. The only thing that is really affected is the code that could use AVX512_FP16, but I doubt there's a lot of it outside of Intel.)