During the WWDC keynote and State of the Union, Apple presenters were very clear in stating that Apple is developing a family of SoC's specific to the Mac and that they won't use processors destined for iPhone or iPad.
Apple said the first. They really did
not say the stuff after that "and that they won't".
If the first Mac out of the gate is skipping Thunderbolt ports, it would be relatively easy for Apple to use the same die with some narrow subset of features turned off/on to both cover the iPad Pro and the return of the one (or two) port , wonder MacBook.
Apple can easily stamp a different label on the chip package. But if limited to just two USB-C ports and sub 32GB RAM Apple could probably relatively easily cover both with the same die. Just some specific Mac features turned on and slightly different boot firmware. ( The first phase of boot is highly likely the same with iBoot. ).
If the objective is not to "crush" the MBP then they could access a "lightest" , "thinnest " ever mac with a Mac SoC that was the lowest in the power draw curve and limited ports. Also save a chunk of money because basically reusing the same die in multiple product. ( That die reuses is pervasive across the rest of the Apple SoC usage. iPad Air 4 is using same A14 as iPhones. etc. etc. etc. ). The A12X and A12Z are
exactly the same die. Just different parts turned on/off. There is a different name stamped on the outside. This tactic will probably get used more often when the Mac SoC packages come out.
Further up the product line up, yes it will be a different die. But Apple never promised there would be zero overlap. In fact, somewhat quite the opposite of bragging about how the 12Z is faster than average Windows laptops sold. Which means a Mac laptop with a successor of the A12Z ( i.e., A14X/Z ) would be faster than most laptops . They could still brag about how it was a "rocketship" fast and super duper , ultra thin. ( Which has been the great white whale Apple has been chasing the last 10 years in their Captain Ahab quest. )
The second coming of the MacBook ( 2015) was a one port wonder like the iPad. Apple spent too much time and effort to bring that model into close alignment with the iPad Pro to probably walk away permanently. The lack of a reasonable fan/cooler in the lastest MBA is another indicator that Apple is 'itching' to do some lowest of low power Mac laptop model also. It would be very "Apple style" for them to launch with a system where they said "see this is why. we couldn't have gone this amazingly thin if we didn't do a utlra custom SoC".