I haven't been paying attention to the engineering particulars; I just know that the two laptops and the Mac mini released have no user upgradable RAM. Are you saying there is something about these "M" chips that suggests there will be no upgradable RAM?
It was written in the assumption that Desktop M chip will have much bigger GPU and hence higher Memory bandwidth requirement.
They could use 8 Channel DDR4 which gives them 200GB/s. That is roughly 3 times the memory bandwidth of current mobile M chip. Or DDR5 and pushes it to 400GB/s. Although DDR5 is barely out.
Also worth noting, if they use 8 Channel DDR configuration, the minimal amount of RAM shipping would be 64GB.
None of the above hypothesis seems very Apple to me.
I'm missing something.