The problem is, many LCDs have vertical and horizontal axes swapped, and this is typical for most small screens. And small screens are especially relevant to platforms such as Jetson Nano.
If the intention is to limit resolution to 4K, perhaps the check could be changed like this:
if ((mode->xres > 4096 || mode->yres > 2160) && (mode->xres > 2160 || mode->yres > 4096))
Then modes beyond 4K which can crash the system (I guess something like 3840x2560 or higher) will be filtered out but both landscape and portrait LCDs with 2.5K and 4K resolution will work out-of-the-box.
As it is now, it’s strange to see 3840x2160, 2560x1440 and even 1080x1920 supported but not 1440x2560 and 2160x3840. Especially considering the fact it is possible to use 3840x2160 in portrait mode with Jetson Nano, or 1440x2560 in landscape mode, etc. As far as I can tell, only noticeable difference between portrait and landscape LCD is default orientation, but it can be easily changed (for example, by putting something like sleep 5; DISPLAY=:0 xrandr --output HDMI-0 --rotate left in ~/.xprofile). And for some projects, LCD with native portrait orientation may be a perfect fit.
By the way, I googled CEA 861-F and it is labeled as “DTV Profile For Uncompressed High Speed Digital Interfaces”. For DTV screens which as far as I know always use landscape orientation, it makes sense to talk about vertical 2160 limit. But this does not apply to mobile and embedded platforms which can have either portrait or landscape screens. Hence the suggestion above, still use 2160 limit, but in a way that will work for both landscape and portrait LCDs, this would make Jetson Nano much more user-friendly. Besides, many projects would work just fine with stock kernel, so not everybody needs or wants to learn compiling and patching the kernel, especially right after they purchased Jetson Nano and LCD for it and trying to get started. Don’t get me wrong, I think that Jetson Nano is great, but it could be even better if some small but annoying issues such as this are fixed. So please consider my suggestion above, or something equivalent, to allow using 2.5K and 4K portrait LCDs out-of-the-box.