The descriptions in verse 7 of the lamb-skin and the blood were indications that the Gadiantons wanted their physical appearance to be intimidating. Thus, when the Nephites “had all fallen to the earth, and did lift their cries to the Lord their God,” the Gadiantons interpreted that event as showing that their desire to intimidate had been accomplished. This emboldened the Gadiantons.
Mormon makes certain that we see the contrast. The Gadiantons were joyful because they had misinterpreted the Nephites’ actions. He tells us that “in this thing they were disappointed.” That dry foreshadowing informs Mormon’s readers what is coming next, when the Gadiantons rushed to the attack, but the Nephite’s received them “in the strength of the Lord.”