[namibj]

Actually they currently just put a retimer chip on the backplane, attach the NVMe drives to the otherwise passive (as far as high-speed data goes) backplane, and use QSFP+ twinax cabling between the backplane and a sometimes passive "HBA" card that adapts from PCIe slot to this QSFP+ cage. Afaik they can run PCIe3 x4 per cable. If you look for open compute hardware, you'll find various modular blade-like designs.