Steam

Indeed. They were upfront about it at least. You are literally using their inhouse firmware tool that isn't publicly available. After sending the request, I had the files and guide same day. I used my work laptop for it and it took 5 minutes. Easy peasy.

This is the model I got: https://www.cablematters.com/pc-1485-126-usb-c-multiport-adapter-with-8k-hdmi-2x-usb-30-gigabit-ethernet-and-power-delivery.aspx

Article/Blog Post: https://www.cablematters.com/Blog/HDMI/how-to-get-4k120hz-with-mac