Not all cycles cost the same amount of power. (Not that you would want to spam nops for thermal management, you should idle the core with a pause etc that actually tells the processor what you are trying to do.)

It used to be the case with intel macs and their atrocious confluence of cooling system, thermals, and power supply system (the CPU actually was not really to blame).

But when RAPL and similar tools to throttle CPU are used, the CPU time gets reported as kernel_task - on linux it would show similarly as one of the kernel threads.