Master Sun, with Shang Qilong's memories, had a relatively comprehensive basic understanding of the Heaven Realm. With just this information, Master Sun believed he was far ahead of the Great Sect Leader and King Monroe.
However, there was no doubt that Shang Qilong's memories were limited regarding the internal power distribution of the Immortal Race, the opposition between Master Amos and Gerald and Howard, as well as the comparison of their respective strengths. This was because, according to Shang Qilong's personality, he would only selectively believe the information that was beneficial to him. For Master Sun, this was a kind of misdirection.
So, Master Sun still needed Historian Gu's immortal soul to obtain a more objective understanding by comparing the two.