That is true - you can physically jam more m.2's into a slot; however, you could conceivably get one of these ridiculous things to run enterprise drives:
Expansion: Dual U.2 SFF‑8639 interface to PCIE 3.0 X16 SFF‑8639 desktop drive expansion card. In order to read the two hard drives normally. Drive Free: You can install the fixing screws directly after inserting the interface, no drive and no data cable.
www.ebay.com
It only does PCI-E 3.x but that's what most of the cheaper enterprise SSD's are anyway.
I'd had also wondered if the slot can provide enough power to run four drives at once under max load but it looks like there is a SATA power port on the back of the card.