The PCIe slot is not for connectivity but for 75 watts power with the 6 pin PCIe.
Looking at it now it is an easy way to have 16 M.2 drives in one place but would have to connect to a RAID card which means 2 slots would be taken up to make any use of this card. Each SFF-8643 can connect to 4 of the drives and that is why there are 4 of them which means 16 independant drives controlled by a 12Gb RAID card into a 16x PCIe lanes.
1x 6Gb per 4 M.2 (drives speeds upto 1.5Gbps) This would help on trying to saturate the PCIe bus for testing purposes.
PCIe 16x gen3 is limited to 16Gb/s but not every drive would be written to at the same time unless a PCIe 16x gen4 RAID card comes out.
Could be used with https://www.scan.co.uk/products/4-port-highpoint-ssd7120-4x-dedicated-32gbps-u2-ports-to-pcie-30x16-raid-controller