Just checked: looks like it does provide a full size pcie slot. My guess is that the intended use case is for developers testing compatibility. You could almost certainly plug in an nvme drive through an adapter to that full size slot (although booting from it would likely require a custom uboot build) but if you use up the one slot for storage, you can’t plug in any other peripherals to test.
It's a 16x slot, but it only provides 4 lanes of PCIe 3.0
Appears to be a SoC limitation, which only has a single 4x PCIe 3.0 interface, and doesn't appear to support bifurcation (splitting into two 2x slots, or four 4x slots). You could probably throw a PCIe switch on it, but PCIe switches are expensive.
The SoC wasn't designed for this product, it was designed by 3rd party company for computer vision/robotics/manufacturing use cases.
Yeah, the Intel Horse Creek chip with the same SiFive P550 cores was probably much better on I/O -- and likely DDR IP as well, if they used their good stuff -- but for some reason Intel decided not to go to mass production on them, despite showing off a working board at a trade show in IIRC September 2023.
