Flunra must have had knowledge of past events personally to be precisely correct.
It somewhat made Adhara wonder what kind of person he was back then as he even knew the dispute between the Demons and the Fire Elementals. He was one of the guardians of the Dark Prince, but he must also act as an informant to know this much.
Maybe he was tasked to keep tabs on important races and report back to the prince.
Nevertheless, he was right about the Fire Elementa's motives.
By annihilating the Rastrikan Demon—a malevolent force responsible for numerous atrocities against not only the Fire Elementals but also various other races—Rex effectively eradicated the primary problem that plagued the Fire Elementals.
A problem that may as well caused them to hide inside the crust of the earth like this.
Due to that, the Fire Elementals wanted to show their gratitude.