This article is a badly assembled collection of other people's ideas. Taking most out of the content out of its original context is why it doesn't make much sense.
I agree. It seems very heavy on pictures, and light on explanations or reasoning for the rules given.
I find a much better resource is "Butterrick's Practical Typography"[0], which is a very accessible. If you want just the absolute bare essentials, the first section of the book/site "Typography in 10 Minutes"[1] is fantastic.
