It's a weird fact of the modern PC industry, but the motherboard "chipset" or "Platform Controller Hub" (to use Intel's terminology) is basically just a PCIe-attached I/O hub that exposes additional PCIe, USB, SATA, and platform services. Contemporary CPUs are built as SoCs, so all the essential functions to run the machine are integrated into the CPU itself. That's why companies have started creating what are effectively I/O breakout boards by slapping motherboard chipsets onto PCI Express cards, including the one pictured above from WisdPi, as well as one coming from Minisforum, as spotted by PC Watch at Computex.
In the case of the AMD Promontory 21 chipset, it really is just a PCI Express I/O controller. In fact, AMD created the X670 chipset by simply wiring up two Promontory 21 chips in series. It's the exact same chip, just used in pairs on the fancier motherboards, so you end up with piles of I/O, though it's all running over the same PCI Express 4.0 x4 link to the CPU.
