As one of the earliest higher ascendants to become a grand lord, Valkries' words naturally carried a different weight. She was also the only one who could maintain her normal conversational tone when she talked with the King, as if there was no difference in class between the two. More importantly, the King had never shown any opposition, which was enough to prove the point.
A conclusion drawn by the Nightmare Lord naturally carried its own persuasive power.
Moreover, Hackzord had indeed discussed the possibility of the humans' inheritance of a legacy with her. Although she didn't say it personally, everything else were reasonable assumptions. He also intentionally brought the matter up after he showed the Western Front's battle situation from his memory. Perhaps the King might ascertain his claim while he was reading his memories, but it was unlikely he would verify this matter specifically.
