Just because two guys got together a hundred years ago and wrote some stuff doesn't mean it's worth dedicating a life of writing to.

Let alone decades of arguments supporting the claim that their style guide is at best only useful for a small subset of writing, the two themselves admit that there can be no one universal styling guide in a variety of ways. You can see many examples in the text itself in which the authors seem to forget their own advice.

Consider:

Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences, for the same reason that a drawing should have no unnecessary lines and a machine no unnecessary parts. This requires not that the writer make all his sentences short, or that he avoid all detail and treat his subjects only in outline, but that he make every word tell.

Which could communicated much more succinctly containing exactly the same information without any extra exposition:

Vigor is concision. A piece should contain nothing unnecessary, just as a drawing has no unnecessary lines [needless repetition](and a machine no unnecessary parts). This requires not that all sentences short be short, but that every word tell.

Or even more succinctly with only the actual message:

Vigorous writing is concise. Concise writing is vigorous (If you're willing to be charitable enough to provide a second example)

That this does not require unnecessary brevity is easily inferred given that the word is "concise" meaning "free from all elaboration and superfluous detail." not "brief" meaning "short". That the writer should follow the advice is made plain by it being presented in a book of advice. The first two sentences alone (if you grant that the second sentence is necessary) contain four repetitions of the same information. If "vigorous writing is concise" then why have we said the same thing five times?

It is an error to apply those style guides blindly to mismatched contexts. Other than as an exercize in following a style guide, it is not great to teach students that they should always write in a journalistic style, because it is simply untrue. There is nothing wrong with writing "A program will be written" when it is unknown who will write a program, and it is an error to avoid the passive voice by adding incorrect details.