Level 5 came out of the Good to Great research, which asked whether a good company can become a great one. Collins was not looking for a leadership theory. He was comparing companies that made the leap against direct comparisons in the same industry, at the same time, with the same opportunities. The leadership finding fell out of that comparison, and it surprised him.
Three things in that sentence do the work, and the popular version usually keeps only the first two.
Humility is the one everyone remembers, and the one most likely to be misread as softness. In the research it showed up as leaders who were often self-effacing, sometimes shy, frequently short on charisma. That describes how they presented, not how hard they pushed. Will is the counterweight, and Collins calls it indomitable for a reason. The humility governs who gets the credit, not how much gets demanded.
The third piece decides the level. Ambition channeled into a cause bigger than you are. A level 4 can be enormously capable and enormously ambitious, and the ambition terminates in himself. Same drive, different destination. The comparison companies had leaders like that, at the same moment, in the same industry, and they did not make the leap.
Position in the sequence matters too. Collins puts level 5 and first-who-then-what before the hedgehog concept, with disciplined action and the flywheel after it. Leadership comes first because everything downstream compounds in whatever direction it is pointed.