The Lord of the Universe wouldn't possibly assign Ling Tian a mission to fight Chi Xue and the others to the death while he was still recovering from his rebirth after consuming the Phoenix Soul Fruit and hadn't yet returned to his peak. That would be sending him to his death for nothing, and it was almost impossible. Such a mission would only be possible once Ling Tian had recovered to his peak.
Of course, that also depended on another factor: whether Chi Xue and Po Tian were at a higher level than Ling Tian. If they weren't, then it would mean they were no match for him and couldn't pose any threat to him at all. In that case, there would be no point in using them to temper Ling Tian and improve his strength.
By now, after all this time, Ling Tian had returned to his peak, while Chi Xue, Po Tian, and the others had broken through to the late stage of the Near-Saint Nineteenth Heaven, putting them above Ling Tian in cultivation. If the Lord of the Universe intended to assign them a mission to fight to the death, now would undoubtedly be the perfect time. Yet even now, there was no sign that the Lord of the Universe planned to give them such a mission. This led Chi Xue and the others to further conclude that he never would.
It made sense. Their past experience with missions showed that the Lord of the Universe usually gave them some advance notice—say, a hundred thousand years, or at least ten thousand. Yet some time had already passed since Ling Tian recovered his cultivation, and the Lord of the Universe still hadn't assigned a mission. That was enough to show he wouldn't be giving them a mission to