When you strip away all the stuff a laptop doesn't need, you're left with... an x86 chip!
You're left with an ARM chip
FaceID is not solely dependent on ML, it's also managed by the secure enclave co-processor which is also used for Touch ID which is available on Macs now. ML helps to reduce false positives.
Apple's T2 chip is an ARM-based processor that's already in almost all of the newest Macs, it is used as a storage controller (which allows Apple to encrypt the drive very fast and transparently), security enclave processing (touch ID on MBA), Siri processing, and more. Every year, more and more of the processing is moved to Apple's T series co-processor.
Apple's custom silicon allows them to integrate software and hardware on a deeper level. Intel develops CPU for the mass market. Apple develops for their own customers only.
With Apple's focus on on-device ML, I would guess this will be the first part of the A-series trifecta (CPU, GPU, Neural) to be included on a Mac and exposed to developers. I can imagine a bunch of possibilities for such a chip, not just FaceID.