Wan Shengjian was the last disciple of Heavenly God Ancestor and was a honorary disciple. The truth was that he became a disciple after the Heavenly God Sect defeated the Cleansing Incense Ancient Sect. During that time, Heavenly God Ancestor was already in isolated cultivation, but because of Wan Shengjian's great talents, he broke the rule and took him in as a honorary disciple.
In reality, the amount of time Wan Shengjian was taught by Heavenly God Ancestor was very little. His older brothers taught him the dao in place of their master. However, Wan Shengjian's talents were indeed amazing. During the Difficult Dao Era, he still broke through to the Ancient Saint realm and approaching grand achievement. If it was not for the horrible nature of the Difficult Dao Era, he would have been an unfathomable Heavenly Sovereign.