Two Sets of Rules, the Good and Bad?

[MaxWilson]

They're not using "Should" as a moral thing. They're using it as a practical thing.

Put another way, when you want to cut the beef you just cooked, do you use a knife, or do you use a laser? Both work just fine (assuming a sufficiently powerful laser) but one is a lot more practical, convenient, and easy.

"[A] DM should never use PHB rules for NPCs" is also not a moral claim. It's an (fallacious) practical claim. It's like claiming that a knife is a sufficiently-good substitute for a spoon that you shouldn't buy spoons, just knives.

What they really mean is "I personally don't like using PHB rules for NPCs; they don't fit my style of game" but then they overgeneralize.