Aren't those 400 GB/s a figure which only apply when the GPU with its much wider interface is accessing the memory?

That figure is at the memory controller.

It applies as a maximum speed limit all the time, but it's unlikely that a CPU would cause the memory controller to reach it. Why it's important is that it causes increased latency whenever other bus controllers are competing for bandwidth, but I don't think Apple has documented their internal bus architecture or performance counters necessary to see how.