This Half-Black Dragon Lizardfolk was less concerned about the life or death of the Lizard Queen and more worried that if she were not thoroughly dead and one day reemerged, and they had already betrayed her, it would undoubtedly be a disaster for the entire Tun Marsh.
"She is indeed dead, but I have control over the Dragon Soul. Without the Dragon Soul, even the Dragon God cannot resurrect her. If she wants to come back to life, it must be with my consent." Gaven did not intend to hide this fact, knowing that once they joined his camp, they would sooner or later learn about the situation at Weizemay Manor, "As long as you don't betray me, I guarantee she will never have a chance of resurrection, unless you wish it to happen."
Gaven was just using words to appease them, even if they desired it, he would not do it, because the cost of raising the Lizard Queen was simply too great. Eating so much every day was akin to devouring Gold Coins.