After more consideration it is much more likely to be LPDDR4X just like the recently released A14 chip. It would seem unlikely that they would have developed a brand new memory interface.
My guess is that they are using up to 2 HBM2 memory stacks (from the picture). Each is limited to 8GB . If they were to go to HBM2e in M2 they could get up to 2x24GB. The biggest advantage of HBM is lower power per bit as the signals are all on the package running at a lower frequency.
The memory market is getting fragmented with Nvidia having seemed to moved from HBM2 to GDDR6X (Micron being only supplier). LPDDR super low power (cell phones) and DDR4/5 for rest of market...