They could put a tiny microcontroller that has a USB HID host and translates to PS/2 connected to the FPGA. So you have a USB socket on one side, you plug your normal USB keyboard or mouse into it, then that socket is connected to the microcontroller, and there are two wires (PS/2 clock and data) connecting the microcontroller to the FPGA.
That's a nice idea. I suspect they must really be trying to keep the BOM cost down though.
I was struck by just how little there is on that board other than the Spartan-6 FPGA; which looks to be one of the smallest versions of what is now a 10 year old design.
There's one SOP/SOIC-8 package by the FPGA which I assume is configuration flash. One 50MHz crystal near to that. Some power regulation on the right-hand side of the board. One Alliance SRAM package. And that's it other than the passives and connectors!
I guess it's possible there's some interesting stuff on the other side but I'm doubting it.
EDIT: Wow I see they are selling this for GBP 300; which is over USD 400. You really aren't paying for the hardware; this has less content than a USD 70 dev board.
You can assume that USB mice are going to become increasingly unlikely to work with those as time goes on - they're just passive adapters that instruct the mouse "hey, someone is trying to plug you in with an adapter to a PS/2 port" and hoping that the mouse knows how to switch protocols.
As PS/2 becomes a deep legacy standard, the likelihood is that mouse manufacturers will simply stop bothering to include that capability.
Some hobbyist and educational FPGA dev boards do that, because handling PS/2 on the FPGA is so much easier than raw USB. Here is one example: https://reference.digilentinc.com/reference/programmable-log...
"The Auxiliary Function microcontroller (Microchip PIC24FJ128) provides the Nexys A7 with USB Embedded HID host capability. "