It does work and has worked for over a decade. You can configure scaling under settings in Cinnamon or plasma for instance or via environmental variables in a simple environment like i3wm.
The post is from the Dev of i3wm an x11 window manager complaining among other things about how well his 8k monitor works under x11 and how poorly it works under Wayland.
You can also consult the arch wiki article on high DPI which is broadly applicable beyond arch
In that time I've had Hidpi work perfectly on first on Nvidia then recently on AMD GPUs on several different distros and desktops all running on X on several distros. They all worked out of the box and were able to scale correctly once configured.
The totality of my education on the topic was reading the arch wiki on hidpi once.
AFAIK one cannot span one x session across multiple GPUs although AMD had something that it once referred to as "eyefinity" for achieving this.
It is rarely needed discreet GPU often support 3 or even 4 outputs
One may wonder if you tried this a very long time ago back when AMD sucked and Nvidia worked well in 2005-2015
The post is from the Dev of i3wm an x11 window manager complaining among other things about how well his 8k monitor works under x11 and how poorly it works under Wayland.
You can also consult the arch wiki article on high DPI which is broadly applicable beyond arch