This particular lecture was about the origin of "blackboard grammar" or prescriptive grammars that most people in the English speaking world would have been raised with. Mostly prohibitions like those against double negatives or starting sentences with short conjunctions (FANBOYS). If you want the SPOILER version, a lot of it came from feeling that Latin and Greek were noble, well-educated languages and English was getting "dirty", so we needed to make English more like Latin and Greek, and we need to stem the change in the language. For an example of the last of those, there was the pronoun agreement table. There was a change taking place at the time to give singular you a singular agreement, but two writers got this practice (which actually makes more sense than singular you a plural agreement) demonized so that now this is viewed as low-class and uneducated.
An example for the former is the so-called rule about splitting infinitives. The reason this was prohibited by these two textbook writers (that influenced English education for centuries and extends into today) is because in Latin, you literally cannot split infinitives because they are single words. So having separate words to accomplish this instead of the lovely Latinesque way of doing it all with inflection is inferior. That being the case, the least we could do is not allow them to be split like barbarians would do. We need to imitate the elegant, intellectual Latin and Greek instead of the (illegitimate child) of a language English had become. The conclusion is that this sentiment toward language is not natural or reasonable.
Watch this movie なう。 It's legal, free... And it's more than its premise. It's not saying Fast Food is good food. Just watch it.
Legend of Crying Bronies: Twilight's a Princess