That free markets ensure a Pareto optimal allocation of resources is the first fundamental theorem of welfare economics. It does not imply any normative statement on whether the distribution is "just", as that is very difficult to qualify.
There's quite a bit of difference between pareto optimal and "most effective". In many situations, pareto optimality is actually among the worst of all possible outcomes (see the prisoner's dilemma, also local maxima). If you had used the former term in your comment it would have been more clear. As it is, it reads like a deliberate attempt to invoke poe's law.
