That’s what they want you to believe. However, in practical terms, no one’s going to sue authors who say “Let’s xerox those pages,” mostly because that’d be playing with dynamite. The absolutely last thing they want is a court to rule is “since ‘coke’ is a general term for soda in many areas of the South, the author’s use of the name is generic.”
The purpose of the Writer’s Digest ads is not to stop people from using the words as nouns and verbs (and, if you notice, the ads don’t forbid the author to do so, but merely ask politely that they refrain from doing so), but to be able to prove, if challenged, that they were working to defend their trademarks (an important factor in keeping it from lapsing).
If you use a trademark generically in a work of fiction, the most you will get is a letter from the company’s lawyers reminding you of the trademark rules. Again, this is to cover themselves so they can show they are defending their trademarks.
It is different if someone was trying to set up a business; the trademark people would be on you like a ton of bricks. But to my knowledge, there has never been a case of an author taken to court over the misuse of a trademark. It’s too much of a risk for the trademark holder.
My SO has had to change names of cereals in one book. He also had to change a character who was just too close to a real life character because the editor was concerned. In general, we make the names up or use a generic term – “The boy ate a peanut butter sandwich” as opposed to “the boy ate a Nutella sandwich”.
Place names are fair game though although the high school example is a bit close to the bone and probably would not get past an editor or lawyer.
Write the book and then worry about it though. These are fixable at an editing stage.
Yes and no: The Jello company likes it when someone who wants a gelatin dessert asks for Jello. But they don’t like it when someone asks for Jello, but the waiter brings him some other brand of gelatin dessert. Your example would be one of the ones that the company wouldn’t like: Here Xerox is spending millions on advertising, and yet it’s Canon who actually sold the machine to the office. What’s Xerox supposed to like about that?