I think you’re absolutely right, and I might have abused the word ‘cool’ (my clickbaity self lol). The rules are more conservative than they are cool.
Two things though. First, if you follow the rules, you will be cooler than nearly all around you, unless you’re in a high-end venue, in which case you’ll just look good. Second, because being cool (in the high-end-venues-clientele sense) requires strongly integrated norms/a lot of experience and changes with trends heavily, it is much harder to teach, and the quickest way to reach that point where you know breaking a rule will make you look better is to start with the rules.
Especially now that the world is so globalised, yes, I think the advice is applicable worldwide, despite perhaps a few differences here and there depending on the country.
The advantage of the rules is that they are less attached to the waves of fashion than purely trendy people are. In 10 years, if you’ve applied the rules, you should look at pictures of yourself now and think ‘that was good!’. Masculine fashion moves slower than feminine fashion. When on top of that you add that the rules change even less; yes, the advice holds through time really well.
I can think of a few changes in the rules, in the past 10-20 years. For instance, minimalist sneakers worn with a suit (and anything else) are now acceptable; streetwear and techwear clothes are getting more and more common (for example no one will think a cargo pant is odd or too ‘technical’ nowadays). There is a slow tendency for things to go more casual, so it’s the formal rules that progressively stop being respected. But it takes a while.