I have no familiarity with this alleged expert on manners, but I’ll say that @Exapno_Mapcase makes an excellent point. Real manners is about making people feel comfortable; fake manners, whether you call it “etiquette” or something else, can be nothing more than artificial pretension intended to elevate oneself by snubbing one’s perceived inferiors. To me the key distinction about manners is the extent to which it accomplishes a useful and desirable purpose, whether it’s making everyone feel at home, or some other useful purpose.
On that note, just one quick comment on this bit from the OP:
This isn’t something I’d ever get hung up about, but it is actually one of those little things that usually do make sense. As recently discussed in another thread, one of the factors that helps make food more enjoyable is presentation. Presentation can be quite elaborate at a formal dinner, or much less so at an informal one, but even then, presenting someone with a dish that has been nicely laid out is part of the appetizing appeal, even if it’s just a backyard barbecue and not a French chef sprinkling exotic garnishes and drizzling swirls of fancy sauces.
But here’s the thing about cutting only what you’re about to eat. First, there’s a practical purpose in that it allows the rest of the meal to retain its heat longer. Second, there’s an aesthetic purpose as it preserves the original presentation longer. Whereas cutting things up in advance destroys that presentation and makes the dish look – and I say this with all respect to my canine compatriots – look more like a dog’s breakfast.
Still, if that’s the way someone wants to consume their meal, I certainly wouldn’t take offense, and I’d even defend them against “Miss Manners” on the basis that how someone prefers to eat their meal is their own business. But I’ve explained why IMHO there’s some valid reasoning behind the advice you read.