I disagree, the Dunning-Kruger effect shows that those who think and talk the most highly of themselves are generally not the best whereas those who often state that they're not or rate themselves below the upper tier have more ability than they state.

Those who "know" they're the boss(and profess it) are actually far more likely to not be.

Essentially, he who talks the highest of himself is often boasting while those that are genuinely the highest often don't rate themselves as such.

"I think I'm the boss" can be interpreted two ways. It can be interpreted the way you've interpreted - that he is uncertain of his bossness. Alternatively it can be interpreted another way using split realities.

"I think I'm the boss"... Meaning, I believe myself to be the boss and am confident within myself, I will not however push that upon you, it is for others to judge and decide for themselves that I am or not.

Essentially it's being the boss of your own reality while accepting that in the shared reality we all have it is impossible to be the boss 100% of the time.