In this article the distance calculation that is made is for distortion on the pixel lines. That is the only consideration that the article is making. The other consideration should be the peripheral vision. If you are placed so close to the TV yours eyes will be shifting constantly to view the area of view which will make it difficult and unpleasant to your vision. This should be your first consideration. After considering that one can make the other considerations to make your viewing a pleasurable and complete experience.
Okay, this entire guide is silly, and I have no idea where you got your numbers from.
How far away people like to sit away from the TV is entirely subjective. I have a 65 inch 4k TV and I sit 4 feet away from it. According to your guide that's way too close, but to me it's perfect. Why? Because the bigger the screen is to your face, the more immersive it feels. If I'm watching TV and most of my vision is filled with TV screen and I can see everything on screen, to me that's perfect.
I also use a 41 inch TV as a computer monitor, which I sit 2 1/2 feet away from. I always laugh to myself when people say they have a "big" 27 inch monitor. I don't understand why people like tiny monitors, especially when TVs these days are so cheap. Web browsing, video streaming, and PC gaming is so much more enjoyable on a bigger screen.