I don't think you read the article. It is only saying that there are two things you need to know in jest. If you looked at the article, you'd see that many fields, such as CS, have more than one set of "two things" that you need to know.

I will not comment on whether most fields can really be boiled down to just two notes, but you'll notice that the author states that one of the "things" about his rule of two things, is that everyone you ask will give you a different two. Since the multiple CS entries are all (I think) from different people, they do not violate the author's assertion.

I think it's meant to be humorous, perhaps a light-hearted source of insight.

Boiling complex fields down to flippant levels of brevity helps the summariser to develop deeper understanding. It can also save a lot of time.

See also: "Desert island" lists of books, movies, music, art, software etc.