Doesn't seem much different than various multichip or multisocket solutions where different parts of memory have different latencies, called NUMA. Basically the OS keeps track of how busy pages are and rebalances things for heavily used pages that are placed poorly.

Similarly, Optane (in dimm form) is basically slow memory, OSs seem to handle it fine. NUMA support seems pretty mature today and handle common use cases well.

With all that said, apple could just add a second CPU to double the ram and cores, seems like a great fit for a Mac Pro.