It was true that the girdle of Dragon's Peak stopped them from flying away from the mountain over a certain radius. It was a spell that had taken the elder wand's full strength and nearly broken it. The wand still bared a crack, such as the power required to trap the dragons behind an unpassable veil. No doubt, Temenos had tried to escape and felt his strength fail him as he momentarily surpassed Dragon's Peak's girdle. This may account for the beast's subsequent anger and vengeance upon the giants. Albus explained to the dragon that it was better this way, that the world below was afraid of dragons and would try and hunt them down should they leave the safety of Mount Khadur.