The standard that a court uses when considering a motion to dismiss (e.g., Rule 12(b)(6)) is to first assume all of the facts alleged in the complaint are true. The judge then decides the motion as a matter of law. That is the exact opposite of weighing the evidence, which doesn't happen until trial.

Edit: I think we're in agreement that "on the merits" here is being conflated with a trial that considers the probative value of evidence. They are not the same thing.