Every M-series Mac has shipped with completely standard LPDDR4 or LPDDR5(X) memory chips. While these can be a bit more expensive than socketed non-LP DDR DIMMS, we're talking maybe 10% more expensive, not 1000% more expensive (which is what Apple charges for upgrades vs standard retail price for DIMMs).
Apple's marketing department would be happy for you to think otherwise, but the "secret sauce" of their high memory bandwidth is completely due to having more memory channels built into the SoC than a standard x86 CPU.
Apple's marketing department would be happy for you to think otherwise, but the "secret sauce" of their high memory bandwidth is completely due to having more memory channels built into the SoC than a standard x86 CPU.