There are many things incorrect about that blog's approach. As others have pointed out, this isn't like HD Audio, where people literally cannot tell the difference between normal CD quality and HD Audio in any realistic test. I can absolutely see the difference in a 4K screen, and I can tell if even a smallish laptop screen is 4K or not from much higher distances than normal usage. I remember the first time I saw an Apple Retina display on a tiny laptop from across the room and said "holy shit!" out loud and walked over because I could see how sharp it was from meters away.

First of all, 20/20 vision is the average, not the best. Many people have substantially better than 20/20 vision. I remember laughing that I could read the super fine print "copyright notice" at the bottom of the eye test that is about 1/3rd the size of the smallest font in the test itself.

Secondly, the eye is complex, and has surprising capabilities that defy simple tests, which are designed as a medical diagnostic tool, not as a test of ultimate resolving capability. For example Vernier Acuity (1) means that much higher resolution printing (and screens) are required than one might think based on naive models.

1) https://en.wikipedia.org/wiki/Vernier_acuity

I don't know what the methodology of that site is, but it's certainly flawed when it comes to computer monitors. Based on their calculator, I shouldn't notice the difference between 1080p and 2160p when sitting 2 feet from my 16" laptop monitor, but the difference is night and day. I don't want to get into a philosophical debate, but if I can see the difference that your equation says I shouldn't be able to see, the equation is wrong, not reality.

4k is more than resolution too, has more colors that can't be shown in regular RGB formats.