In the article, the author uses OpenGL to make sure that they're interacting with the screen at a "lower level" than plenty of apps that were written against X. But that's the rub, I think the author neatly sidestepped by mostly using stuff that's not in "vanilla" X11. In fact, the "standard" API of X via Xlib seems to only expose functions for working in raw pixels and raw pixel coordinates without any kind of scaling awareness. See XDrawLine as an example: https://www.x.org/releases/current/doc/man/man3/XDrawLine.3....
It seems to me that the RandR extension through xrandr is the thing providing the scaling info, not X11 itself. You can see that because the author calls `XRRGetScreenResourcesCurrent()` a function that's not a part of vanilla X11 (see list of X library functions here as example: https://www.x.org/releases/current/doc/man/man3/ )
Now, xrandr has been a thing since the early 2000s hence why xrandr is ubiquitous, but due to it's nature as an extension and plenty of existing code sitting around that's totally scale-unaware, I can see why folks believe X11 is scale unaware.