Nothing but a pile of linguistic ignorance and the authors’ weird peeves. See http://www.lel.ed.ac.uk/~gpullum/LandOfTheFree.pdf
Most of the specific rules were nonsense: among vs. between, passive voice dogmatism, etc.
Unfortunately, these things have become shibboleths that indicate membership in a particular class of people. If you used among vs. between "wrong" in high school or college, there's a good chance that your teacher corrected you, so now most of the educated elite believes the rule to be "right".
Bizarrely, Strunk & White's prescriptive rules, which had very little to do with English grammar historically when they wrote their book, have now—via generations of dogmatic education—become generally accepted by the educated elite. Ironically, a descriptive grammar must therefore now acknowledge the among vs. between distinction, at least in formal English writing.
We must right this wrong! If we shout loudly enough, maybe we can create entirely new shibboleths, and a new generation of elites can look down on the uneducated masses that follow Strunk & White.
On that subject: as a non-native speaker I always wondered if there actually is a grammar (not syle) guide which is considered authorative, as there is for other languages?
No living language has a fully authoritative grammar book, as the grammar of living languages is discovered, not prescribed. That's a part of what linguists do, after all, and they have plenty of debates over precisely how to do it. The overly-simplified grammars taught to language learners as "Correct" are only the tip of a huge, wonderously complex iceberg.
Here's an example of a more academic English grammar:
https://en.wikipedia.org/wiki/The_Cambridge_Grammar_of_the_E...
Note that The Cambridge Grammar of the English Language is almost 2000 pages long.
Yes, we agree, but those pretending they can when they are backed by governmental powers can go pretty far in term of influence.
https://en.wikisource.org/wiki/The_King%27s_English
Just a small sample:
"The 'split' infinitive has taken such hold upon the consciences of journalists that, instead of warning the novice against splitting his infinitives, we must warn him against the curious superstition that the splitting or not splitting makes the difference between a good and a bad writer."
Why should one use a book on English grammar that can't correctly identify English grammar in the first place?
Out of curiosity I googled "we intend this book" and people do seem to use that wording sometimes in introducing their books. Why don't Strunk and White do that?
There's a reason linguists hate S&W. It's really not good.
This reads like plain bad faith. Pullum lists a dozen or so very specific problems with S+W, and goes on at some length about the evidence against each. Any of those problems is more objectionable than the "modifying unique" thing, which Pullum mentions once in passing but doesn't discuss.