I saw it in Learn Vimscript the Hard Way. I assumed it was common knowledge, since I couldn't see any other practical way to use 5j, etc, on a regular basis, but it's apparently less well-known than I thought.
Pre-vim, one would get quite proficient at judging line distances, e.g. 11yy, through practice. Mind you, that was probably helped by text being a fixed size on a serial terminal and bigger than much text today. However, I do wonder if the article is over-promoting movement by lines. Movement commands like ), }, ]], and good old / should be readily considered as well.