He was wrong.
He would not normally admit it, but Hyperion found himself to have been very wrong about the existence he had only considered interesting!
The gaze of confidence and certainty he had when he spoke of profound matters wasn't merely arrogance and not knowing his limits, but they seemed to be entirely true.
He didn't lie about the Primordial Epitaph, he was capable of discerning it when he didn't have the Essence of Reality, and he bound himself by a contract to be capable of easily and quickly producing 4 completed edicts in a short period of time in exchange for the lives of countless Primordial Beasts.
Like any sane being that knew of the arduous nature of not just completing but just forming Edicts, Hyperion was inclined to not believe him! But after the contract and the gaze of this being still shining with utter confidence, he realized he had to admit his wrong perspective and reevaluate this being once more.