If I have to guess, his main party, the Strange Grief ones, already knew that he is not the strongest. But, they knew that he is a capable leader and someone who can make a good decision (even if it's just him randomly choosing stuff lol).
Remember, a leader isn't the strongest member of a party, but the one who can lead and correctly delegate work to the other members according to their capabilities. This also works out well for these strong members so that they can fully committed to their work and fighting prowess without having to worry about chain of command or decision making. Basically, they could go all out without worries.