Correct me if I'm wrong but 400GB/s is ludicrous for CPU memory isn't it?

> DDR5 octuples the maximum DIMM capacity from 64 GB to 512 GB.[8][3] DDR5 also has higher frequencies than DDR4, up to 8000 MT/s which translates into 64 GB/s (8000 MT/s * 64-bit width / 8 bits/byte = 64 GB/s) of bandwidth per DIMM.

Depends. DDR5 provides less bandwidth than Apple's on-package DRAM precisely because it allows larger sizes and user upgradeability. Intel has produced similarly packaged CPUs since https://en.wikipedia.org/wiki/Xeon_Phi in 2010, at least. A small amount of very fast memory is great so long as your workload fits within it. The moment it doesn't, everything goes awry. The best memory architecture is always the one which works for your workload. AMD produced a desktop part - the 7800X3D - which provides 2.5TB/s of bandwidth to 96Mb of L3. And an Epyc part with 768mb of L3 in a similar configuration across 8 chiplets.

Then there are issues of prefetching algorithms, cache eviction and concurrency strategies as well as speed and associativity, TLB size, branch prediction algorithm especially as it relates to pipeline length and flushes, decoder width and complexity and execution units available for dispatch.

There's just a lot more that goes into fast than memory bandwidth go brrrrr. Also x86 (and increasingly ARM and Risc-V) have many SKUs above those Apple offers in sleek packaging. Particularly silicon fabbed for high end desktop workstation or datacenter duty is a cut above what's available via other channels. For the cost of a Mac Pro you can easily afford an HP Z series or a whitebox from SuperMicro with dual sockets, 16 channels of DRAM, and enough PCIe lanes to make starving children cry.