There's a concept, popularized by biologist Jack Cohen and mathematician Ian Stewart, known as "lies-to-children." The idea is that reality is so inherently complex that when you teach a subject, you are always necessarily presenting an inaccurate oversimplification. The hope is that even though the statements you present are technically false, they can at least be an approximation which leads the student to a more accurate understanding.
Despite the "lies-to-children" label, Cohen and Stewart held that the same principle applies to any explanation that a human mind can grasp. We always start with a grossly, oversimplified model of reality and hopefully can move on to models which are more nuanced, complex, and less inaccurate.
In my experience, this applies to martial arts just as much as it does to science or mathematics. I give my students rules and principles to follow, but I periodically remind them that this is just the way to do things for now, that in time they will learn when to break all the rules I give them.
If you analyze the fights of top boxing champions, you will find that many of them break the "rules" of what most boxing coaches would teach as good form ... or at least what they would teach as good form to beginning/intermediate fighters. In fact, I've seen one top fighter or another break almost every rule of "good technique" that I would teach to a beginner. Does that mean that these champions have bad technique and are just coasting on physical attributes like speed or strength? Not at all. It means that they understand the rules well enough to know how and when to break them.
Every variation of human movement presents a trade-off of one kind of functionality for another in a given context. The standard "good technique" for a given fighting art is generally just a collection of design choices to produce movement patterns where the trade-offs work out pretty well for most practitioners. Most great fighters develop their own movement patterns where the trade-offs of one advantage or disadvantage for another suit their particular body, personality, and skill set. If a great fighter fights with his hands down, it's not because he doesn't know better. It's because he is getting certain advantages from doing so which outweigh the disadvantages. Most likely he will have other modifications to his movement as well which minimize the disadvantages and maximize the benefits from that choice.
Since Ali was mentioned, here are some detailed breakdowns of aspects of his technique:
I wasn't talking about breaking "a rule". Ali broke every rule, including defense. His fundamentals were one big rules break. I guess that's why some argue he was the best since he transcended boxing.