Since they did not know where Yggdrasil was yet, Ymir continued to work as the "Speaker" in Athens Sanctuary after the call, leaving Hao Ren to consider the new information he had just been offered.
When he heard Ymir's description of Yggdrasil, Hao Ren thought of a few things, such as how "Yggdrasil" the First Born came to be, and its role during the Mythological Era.
Yggdrasil was powerful — that was undeniable. It could bring the Nine Realms with it and travel through spacetime, covering millions of light-years in just a few seconds. That would required a tremendous amount of energy. Even if the First Born had used some kind of trick to achieve this miraculous feat, it was still a demonstration of great control over power. It begged the question of why Yggdrasil did nothing during the Twilight of the Gods, instigated by the demon hunters.
