I thought it was predominantly the interface?
As digital signals, I thought they were (in essence) interchangeable?
It has been possible to force FreeSync on non-FreeSync monitors (admittedly with hacks) and over HDMI and DVI...
So, it wouldn't be unreasonably to think that it's possible to force FS "out" in a way that can be interpreted by the monitor*.
*Just speculating there.
By the looks of things, there were only ever <10 monitors that "supported" (lots of complaints out there) FreeSync over HDMI.
https://www.amd.com/Documents/freesync-hdmi.pdf
So, it would be pretty foolish to require a highly-specialized monitor IMO, over and above the cost of a 'normal' FreeSync display.... given the vast majority of XB1X users use a TV and not a monitor to begin with.
Not sure on exact numbers, but I'd have to guess that >90% of XB users use a TV.
So, <10% use a monitor.
Of that, it's going to be a tiny% that use one with FreeSync support to begin with....
Then, with very few monitors in existence that'll support it over HDMI...... that market share of users has to be <1% range, comfortably.