It's an M.2 key E connector, not a PCI connector, but it doesn't follow the M.2 standard -- they're just reusing a cheaply available connector. The microcontroller they're using doesn't support PCIe, so it's probably just power, some serial interfaces, and maybe some boot/reset/interrupt pins.

As such, you should have far less of an issue integrating this onto a custom board than a real M.2 card that uses PCIe or USB.