The funny thing of course is that the Steam Machine has DisplayPort, and you can easily get a DisplayPort to HDMI 2.1 dongle for $20 retail. But they are targeting this being a console, and those are hooked to TVs over HDMI so it seems lame to not have a built-in HDMI port.
This is mostly an academic exercise though. HDMI 2.0 does 4K @ 60hz, and Valve have 4K @ 120hz (with 4:2:0 chroma subsampling) working over it too. Given the CPU/GPU in this machine, it won't be able to push higher than those limits anyway.
The more pertinent issue is that many TVs will only do VRR over HDMI 2.1, and many active DP to HDMI 2.1 adapters won't pass VRR through either.
That's also why the Switch 2 supports VRR on its internal display but not when connected to a TV - the dock can't encode a HDMI 2.1 signal. That's just Nintendo being Nintendo though, they could support it if they wanted to.
Only if the adapter is active; passive ones just tell the GPU to switch protocols to HDMI or whatever, so those are still kneecapped by driver limitations.
Edit: I just checked Amazon and active adapters are a lot cheaper (and less niche) than they used to be, though there are still some annoying results like a passive adapter which has an LED to indicate the connection is "active" being the first result for "DP to HDMI 2.1 active".
Club3D makes some dongles that will convert from DP 1.4 with DSC to HDMI 2.1, actually. The only ones I've used personally are physically USB-C (DP alt mode) on the DP end, though. But they make some that are mDP and have DSC support as well, and they might also have one for full-sized DP, although I bet it requires external power.
Edit: The article claims that a good Club3D adapter for this has disappeared. Yeah, there is an old Club3D adapter (CAC-1085) for this and it's not around anymore (and it does require external power!). But it's been superseded by a newer one (CAC-1088) which is still available on Amazon, at least in the US. (And the new one is bus-powered.)
This is mostly an academic exercise though. HDMI 2.0 does 4K @ 60hz, and Valve have 4K @ 120hz (with 4:2:0 chroma subsampling) working over it too. Given the CPU/GPU in this machine, it won't be able to push higher than those limits anyway.