Some calculations. Number from wikipedia. A12X is 62 % larger than A12. An A13X at 7 nm would be nearly 160 mm2 if the same ratio applies and no optimisation of 7 nm node is done. A very larger chip and larger than A9X . No, there is a chance that A13X will use 5 nm just as A10X used 10nm before the A11 at 10 nm was introduced.
Intel is mucking around with "Gen 10" labels for 14nm and 10nm stuff. I highly doubt Apple would go down that twisted marketing labeling hole. if "12" then likely same fab level as other "12' stuff.
As announcement has illustrated, they simply just beefed up the A12X to the A12Z. ( symmetrical 8 GPU cores to match the 8 CPU cores had before. Wouldn't be surprised if that was the original design benchmark anyway and simply throttled back the GPU one 'core' to same a few bucks on wafer costs in the interim. ).
If they "sat' on the same A12 series Neural engine in the 'Z' then this is largely about margin. Shave some costs out of the SoC so that can pay for more expensive camera subsystem and keep the prices the same. [ Won't be too surprised to see this A12Z to show up in a revised AppleTV too. ]