And literally half the bandwidth. NVMe means that you have to write the data, flush the caches over those ranges to ensure that it's actually in DRAM, then instruct the NVMe controller to read it back out of DRAM. With these drives you just write (and maybe flush), and it's done. And they probably have their own dedicated DRAM protocol controller.

And it might even be slightly better than halving the bandwidth needs, since swapping DRAM banks isn't free, so you might be saving on mildly thrashing your DRAM controller when you're using DRAM and the NVMe drive is trying to read at the same time.