Again, the self confidence thing is a bit of a red herring. The peacock implication says to me this is not an inward looking thing, it is not (merely) about your own belief in your own ability, there is something demonstrative about it. Peacocks are flamboyant, they are extrovert. They have self confidence, but the kind of self confidence that can be seen from far away, the kind that cannot be ignored. The kind that has an affect on other people looking at them, not on themselves alone.
It is one thing to believe you are really good at football. Martial has that, as does Rashford. But neither of them are extrovert type players. Neither has that kind of personality that screams "LOOK AT ME" the way peacocks do.
Ronaldo had it, but I think Cantona and Ibra are actually the best examples of it from my lifetime.