You can play the game in a way that makes stupidity impossible, however, it will make genius impossible. Upgrading to the legendary level is having understood how to break the inviolable coaching rules. It is also finding opportunities where other coaches would not even poke their noses. Is it abstract? Well, it is what it is! Welcome amongst Legends!
But that is not all there is. On a more concrete level, the legendary coach has a perfect now or never instinct. For example, when to push for the win instead of protecting the draw, when and how to abandon security to provoke errors, when and how to fully leverage a game situation, and the list could grow on and on.
Legendary coaches usually have the reputation of being lucky, but in reality, they have a stellar risk-taking synchronism, the instinct to spot the pivotal play of the game, the flexibility to immediately change tactics to avoid losing control, and the gift of reading the mind of their opponent.