This doesn’t really answer the parent’s question. That the memory is on a PCB close to the CPU/GPU certainly helps with signal integrity, but it is not by any

This doesn’t really answer the parent’s question. That the memory is on a PCB close to the CPU/GPU certainly helps with signal integrity, but it is not by any means relevant here. The Apple platform has high memory bandwidth compared to x86 PCs because the CPU has a wide memory bus. You can get similar memory bandwidth out of high-end Epyc and Xeon CPUs which use standard DIMMs but with many more memory channels than a regular desktop computer.