How much of PG's blog is based on setting up strawmen and using them to bash on the liberal arts?
> Even one sentence of this would raise eyebrows in conversation. And yet people write whole books of it.
These sentences immediately identify one big, relevant difference between speech and non-blog writing, which is not commented on in the blog post: people do not generally give book-length monologues on a single topic. Books will necessarily end up using more flowery language because if they didn't they would be extremely boring to read.
> perhaps worst of all, the complex sentences and fancy words give you, the writer, the false impression that you're saying more than you actually are.
On the other hand, simpler words and sentence structure give the reader the false impression that you're being more honest than you actually are. Demagogues (especially of the right-wing variety) have known this for centuries: people like simple ideas. Making an idea sound simpler, even if at the expense of actual clarity, means that people will agree with you more readily. That can be dangerous.
But even assuming you're communicating in good faith, sometimes you really need the nuance that only more sophisticated language can grant. In speech, we tend to do this by inflection, body language, and gestures; in writing, those aren't available, so we do it with vocabulary choice and more careful sentence structure. In English (and many other languages), a single spoken word can have dozens of different connotations, or a sentence dozens of meanings, depending on tone and emphasis (see https://bridgeenglish.com/blog/2012/08/28/who-stole-the-mone... for a classic example). In writing, we have to be more precise with the words themselves.
All of that said -
> If you simply manage to write in spoken language, you'll be ahead of 95% of writers
is probably true, but I think it says more about 95% of writers than it does about what's actually good. In most disciplines, the techniques it takes to become "not terrible" are qualitatively different from the techniques it takes to be "good". I would posit that writing is one of those; the best writers are fundamentally treating the written word differently than those of us who just want to get through the day and be understood on a basic level. Moreover, "top 5 percent of writers" is not really that good, considering that most readers are reading the same vanishingly small fraction of writers. Even in a professional capacity, where you're going to read design docs and such from a wider array of writers (as opposed to the extreme power-law distribution of novelists), I'm certain that the top 1 in 20 writers in my company are read way out of proportion to everyone else, and some of them are still terrible writers.