I think it's pretty consistently shown that the people who have legitimate confidence have belief in their own abilities and strengths, but also do not fear being vulnerable or do not dread failure.
Think of it like this, people who view failures as opportunities for growth tend to have a lot better chance at success than people who are so scared of looking like a failure that they hide from things that might take them out of their comfort zone like the plague.
Additionally, when you acknowledge it's okay to fail, you tend to not give too much of a shit about appearing as though you are out of your element or do not quite know what you're doing. And by not caring as much, you end up being better suited to working and improving and shoring up your shortcomings.