That's a fair point to bring up - there
are display devices available these days where it's impossible or at least impractical to follow the
"top of the screen 1-3 inches below eyeline" (potentially more if you use bifocals) rule. Owners of these displays will know if the guidelines work with the display size and adjustments they're dealing with. Something like the
42" LG C3 is problematic for most people, even more so the
ROG PG42UQ, and the
LG DualUp and
Samsung Ark 55 are right out.
With the same exceptions that existed back in the 1980s - children or adults with short stature, individuals using vertically very narrow eyeglasses or bifocals, or with other special needs related to vision or motor function - the guidelines still work with the majority of current office and gaming monitors however.
The U3224KB certainly falls into this category, keeping in mind that the guideline refers to the top-most pixels of the screen - the stuff you're looking at - not the bezel which on this one is a chonker.
As for adjusting these guidelines to each individual's particular situation, the key thing is to ensure you avoid tilting your head down
or up when viewing the screen's vertical extremes regardless of where your eyeline falls vertically. If the monitor's height adjustments or vertical size don't allow that at an arm's length, move the monitor back. If text becomes illegible at that distance, scale screen contents up. If someone doesn't want to do these things because they paid good money for a big screen, that's fine (except for ergonomics) - but perhaps they should consider a smaller screen?