Which Screen do you tried ?
Only since you can't get it working , don't imply other can't either.
It is working like a charm here.
We have never claimed that it is running at HiDPI.
HiDPI is unequal to Ultrawide.
I just told that it is working with 5120x1440 and the fonts are crisp here.
Not as Crisp as on a retina screen (HiDPI) ofcourse where you can run it @ 200%.
To make that work properly you need a higher (vertical) resolution then 1440 px or you have to live with 720px estate in height. But that Screen would cost a lot more than a ultra wide ;)
eg LG Ultrafine has a resolution of 5120 x 2880
That would give you a screen estate of 1920x1080 @ 200%.
So I would need at least 3 of them to get more estate (dual head suxx since you would have always a bezel in the middle of the view)
5720 x 2880 would be lovely :) and then in OLED Quality, THAT would be crisp and enough estate @ 200%
I wonder how much this would (will) cost.