The reason for Duan Ling Tian's temporary concession was simple. As long he and Yuwen Hao Chen successfully overcame the Heavenly Tribulation and became Eminent Celestials, Yuwen Hao Chen would no longer be a threat to him since Yuwen Hao Chen would have already used the technique to overcome the Heavenly Tribulation!
An Eminent Celestial would gain the Celestial Origin Energy that was unique to Eminent Celestials. It was so powerful that even a powerhouse at the Ninth Form of the Saint Celestial Stage who had completely comprehended heaven and earth was inferior.