I believe it's because HDMI-VRR is entirely separate from FreeSync and GSync. Also, I think (not sure) that GSync originally worked ONLY with Display Port. I am not sure if it was the same for FreeSync, but I know now that FreeSync will work with normal HDMI.
It is confusing. But, at least for now, you should be on the safe side if you go with a C9 and a new Nvidia card. However, LG is also putting out FreeSync-capable TVs. I am not sure how their prices compare - but I would have
assumed that they would be less, since that's how it was with monitors. But it appears that what they're doing with HDMI-VRR is outside of the FreeSync vs GSync distinction that normally happens with monitors.
Ideally, you could get a monitor that plays nice with everything, so you're not locked-in on one brand of card. Of course, AMD might have started adding 2.1 HDMI VRR support in their drivers.
Finally, AMD has partially updated their display controller. I say “partially” because while it’s technically an update, they aren’t bringing much new to the table. Notably, HDMI 2.1 support isn’t present – nor is more limited support for HDMI 2.1 Variable Rate Refresh. Instead, AMD’s display controller is a lot like Vega’s: DisplayPort 1.4 and HDMI 2.0b, including support for AMD’s proprietary Freesync-over-HDMI standard. So AMD does have variable rate capabilities for TVs, but it isn’t the HDMI standard’s own implementation.
https://www.anandtech.com/show/14528/amd-announces-radeon-rx-5700-xt-rx-5700-series/2
That is not very promising on the AMD side, but I haven't been able to find more recent information.