Irreducible Complexity isn't just complexity. It is complexity and simultaneity. So the argument is that evolution acts on changes/mutations in a step by step fashion and changes that give an advantage survive and propagate. So A->AB->ABC->ABCD. But what if ABCD confers an advantage but nothing before that does? That is the challenge Irreducible Complexity brings against evolution. Step-by-step won't work since you need to have leaps if there is an irreducibly complex structure.