UEFI doesn't really take care of bringing up the other cores. There's a way to get access to them, but you have to give them back to UEFI before exiting boot services, so that doesn't really help your kernel. You still need to do the SIPI into x86-16 code to really take control of the AP cores.

Even the new way (that isn't actually implemented AFAIK) just SIPIs into long mode code, it doesn't use the UEFI multicore stuff.