Likely makes it easier/cheaper to run DDR4X at 4500 Mhz. Also DDR4x is often dual channel and Anandtech claims the M1 has a 128 bit wide interface in the form

Likely makes it easier/cheaper to run DDR4X at 4500 Mhz. Also DDR4x is often dual channel and Anandtech claims the M1 has a 128 bit wide interface in the form of 8 16 bit channels. So ML, GPU, and CPU duty can have more load/stores in flight and might help explain why the M1 performs so well.