Gideon realizes that it must have been the priests of Noah who had stolen the Lamanite daughters. He was correct, but we do not know if it was only suspicion or if there were some evidence of it.
In any case, it is Gideon’s reasoning that is important. First, he wants to explain the problem to the Lamanite king. Perhaps if the Lamanite king understood that the people of Limhi had kept their oath, but that it was a separate and disgraced group who had done this, the peace might not be destroyed.
The rest of the argument focuses on the alternative, which is an even larger army and even more destruction. While Gideon is attempting to forestall that event, he ties it into Abinadi’s prophecy of doom upon the people. Abinadi had said that they would be “scattered to and fro” in Mosiah 17:17. Gideon sees this current battle, and the possible future engagements, as fulfillment of Abinadi’s prophecies.