Haven't used this app for ages, but yesterday I fine-tuned the HDR settings for my newly bought laptop under KDE/plasma, definitely on Linux. That readme might be outdated (or the tool doesn't work on KDE at all)

My understanding of the landscape currently is that KDE Plasma is the only major DE that supports HDR. I use it for Steam for that reason.

Yeah I think I’m a bit outdated here. But I do think Linux hdr support is weak enough that many Linux-oriented apps are unlikely to do it well