I think the article is honestly a little outdated on this point. The last couple of years the adapter market has caught up pretty well.

The UGREEN 8K@60Hz Display Port to HDMI Adapter I have sitting here supports g-sync (and claims support for freesync).

I mean the article is two days old, so perhaps not that outdated. There also seems to be a difference between G-sync, Freesync, and HDMI VRR.

Why does everybody seem to overlook this?

I'd say because most people don't care about VRR...

People who buy Valve's Steam Machine should care though, because they will get bad frame rates without VRR.

The frame rate won't really be any different, there'll just be a little bit of extra delay until you can actually see the frame that was just rendered, and maybe some barely detectable stutters. Keep in mind we're talking 4K120 or 8K here, the GPU in the Steam Machine is nowhere near fast enough for either of these things. It's likely that there's always some mode with a higher fixed frame rate than what the GPU can calculate, even on Full HD you can probably just do fixed p120 or p144.

VRR is all about smoothness/microstutters, not higher frame rate/performance.

Without VRR you get screen tearing when the frame rate below 60 FPS. (Except if it is locked to 30 FPS.)