In a real world, ignore the theory. Use the diaphragm stop that will give you the depth of field that you require, and that's all you need worry about.
The problem with such broad sweeping statements is, that they are worthless advice in specific circumstances that many professionals and amateurs alike encounter on a daily basis. Granted, some of those are not producing 'artistic images' where blur and lack of resolution can be coined to be a feature rather than a flaw.
Folks have to realise that they are dealing with a photograph and not a laboratory test. Unless they actually do see their photography as a sequence of laboratory tests, in which case, good luck to them, and have fun.
That is also not helpful for those who want to make a more deliberate choice of their camera settings, understanding (all) the trade-offs involved. Mastering one's tools is also part of becoming a better photographer, and it's more fun than being mastered by one's tools. Do you shoot with your camera in Program mode, or do you deliberately choose a shutter speed or aperture priority for a given situation?
There apparently is still a lot of confusion about diffraction, which BTW is the topic of this thread, so if a few laboratory tests can help to improve one's understanding, I don't see the downside of that ...