The author said that the problem is that Apple has introduced a size limit for the display (3360x1890) that is lower than the size of the actual display, which is a standard 4k display (3840x2160).

So 1:1 rendering can cover only a part of the screen, while the remainder remains unused.

If the maximum size limit is used but applied to the entire screen, it does not match the native resolution so interpolation is used to convert between images with different resolutions, blurring the on-screen image.

All the attempts were done with the hope that there is some way to convince the system to somehow use the greater native image size instead of the smaller size forced by the limits.

Nope, you completely misread the post. All Mac’s including M4s and M5s can run at a 1:1 4K resolution all day long filling the screen completely. That’s not what the OP wanted though, they wanted to render at 8k (roughly 7680 px by 4320 px), then downsample that by 2x in each direction to map to the 4K display. Supposedly to make things “look better” than rendering at the native resolution but it sounds insane to me.

That does not seem to be the case for my M4 Mac mini in native "low-DPI" mode with a 4K display, so I think the problem only appears in HiDPI (7680x4320 framebuffer downscaled back to 3840x2160 only). The author seems to be confirming the max intermediate framebuffer is 6720 pixels wide.