All cores are 'on', but unused ones go into low power states so it isn't quite the same thing. Cores in WFI & ultimately retention use very little power so aren't a concern. This state is available to all cores in the system so the A53 cores will go in and out of retention while the screen is on similar to how systems will have hotplugged in the past, expect retention is a much faster state to enter and exit so cores can drop into low power far more often. The hard hotplugging the kernel provides is interesting as a toy to play with (for benching A53's for example), but for practical use the retention states are far more effective than actually doing hotplugging while the device is in use.
Yes, I do realise about this, but trying to explain in simple terms in my previous reply. I assume he just really wanted to know if the disabled cores would be in work. Answer = no (retention or "off"). The "offline" cores won't be available from sysfs either, and any power consumption or work is negligible.