To Lehi's credit it does not dismiss Sariah's fears. Rather than responding directly to her question about her sons, Lehi responds to his label as "a visionary man". It is probably that Lehi had no vision of the outcome of the mission, only a confidence that the Lord would see them through it. Lehi did, however, know about the visions he had received, and he attempts to comfort Sariah with his statement of faith in the visions and knowledge he had received.
One can imagine that while well-intentioned, this would have been little comfort to Sariah, who really didn't want to know whether or not Jerusalem was lost, but whether her sons were lost.