It can't. The spec page says that a single drive tops out around 750MB/s which isn't great. USB 3.2 peaks around 1000MB/s with an NVME drive. So, speed is getting lost somewhere along the line for each drive. With RAID, speed can increase to 2550MB/s, which is pretty decent for an external drive.
Right. This is the same configuration as the OWC Express 4M2. One PCIe 3.0 x1 lane per NVMe with a DisplayPort 1.2 output (OP incorrectly says DisplayPort 1.4).
The reason for only one lane per NVMe is for cost. The single lanes come from the Alpine Ridge Thunderbolt controller directly. To get 4 lanes per NVMe would require a PCIe 3.0 switch with at least 20 lanes which is very expensive.
The point of these enclosures is capacity but it can also do performance with software RAID. With software RAID, you can actually get better performance than with an enclosure with a single x4 NVMe because those usually have poor write speed (down to 800 MB/s for some) compared to read speed (2500 MB/s) while in a Thunderbolt enclosure (Thunderbolt is limited to ~2750 MB/s). With Software RAID, even if the write performance per NVMe is low, it adds up. I don't know of a single NVMe that has good write performance in a x4 Thunderbolt enclosure - these x1x1x1x1 Thunderbolt enclosures can fix that problem. Would be nice to see some benchmarks to be sure.
As for USB 3.1 gen 2 (10 Gbps) remember there are the ASMedia ASM1142 controllers that are also limited to 750 MB/s because of the PCIe connection (either PCIe 2.0 x2 or PCIe 3.0 x1). You need a USB controller with PCIe 3.0 x2 or greater to get the 1000 MB/s numbers.