AMD would be able to do DRAM on package for the lowest wattage "ultrabook" chips, at the cost of producing a very different package for them vs. the bigger laptops that are expected to have upgradable SODIMMs. But I doubt that this "co-location" is that huge for performance. Whatever memory frequency and timings Apple is using are likely easily achievable through the regular mainboard PCB, maybe at the cost of slightly more voltage. DDR4 on desktop is overclockable to crazy levels and that's going through lots of things (CPU package - socket pins - board - slots - DIMMs).

> stacked modules such that the side wall of the Mac Pro is a grid of 4 or more such modules each with co-located memory like the M1 has

Quad or more package NUMA topology?? The latency would absolutely suck.