Not HBA adapters, in fact the design is pretty much directly for use with some very pricy HBAs - NVMe-OF seems obvious target, as I don't see much benefit from

Not HBA adapters, in fact the design is pretty much directly for use with some very pricy HBAs - NVMe-OF seems obvious target, as I don't see much benefit from "just" allowing HDDs in U.2 bay and thus possibly standardising on single fabric bay in server chassis. NVMe-OF has options in form of InfiniBand and ROCE HBA chips that can be configured to act as PCI-E root complexes bifurcated over many many links (in place of default x16 endpoint) with minimal management logic or built-in SoC features. So a network JBOD with NVMe-OF can be essentially one chip + PCI-E links to disks + support/glue logic.