Windows used to let you set the DPI setting of the monitor, circa Win95-WinXP. Software tended to only expect a value of 96dpi, so it wasn't a very practical setting to change, and eventually windows introduced display scaling (which leaves old software in the belief that you are on a 96dpi screen, while allowing software to opt in to knowing about the real device pixels)

Screenshot: https://www.softacom.com/wp-content/uploads/2022/03/11111-10...

I guess you could use that setting on an old version of windows to set a screen to 16dpi, and now a CSS reference pixel is 1/3rd of a real pixel? But more realistically, what the spec is getting at is that a mobile device should set its dpi setting close to the real device setting, and a reference pixel is now something that looks about as big as a pixel on a 96dpi screen. That nobody actually sets their screen dpi on desktops is probably of secondary importance to them