K you missed important context here.
https://wowpedia.fandom.com/wiki/Aqir_and_Troll_War
Unable to properly lay the mountains under siege, the aqir were forced to retreat from the area, suffering significantly from raids and guerrilla actions carried out by the Empire of Zul. Kith’ix, seriously wounded during an engagement with the loa, fled to the northeast with some of its most important aqiri followers. It planned to retire for a time to recover from its wounds, while the aqiri swarm continued its war against the trolls.[9]
Though the trolls had successfully driven the aqir away, the Zandalari were aware that they still posed a potent threat to the continued survival of troll civilization in the future. The insectoids, if left unchecked, would continue to attack outlying troll territories, and it would only be a matter of time before they once more regained to strength to assault the sacred mountains again; and so, at the Zandalari’s behest, the other tribes moved out in force to hunt down the aqir. The trolls soon learned that it was not enough to simply defeat or kill the insects. If any escaped underground, they would establish a new colony, increase in numbers, and rise up again.
The trolls attacked the Aqir after they had retreated to new landa, After THEY started a war of extermination against the trolls.
Millennia later, his body was discovered by curious trolls who promptly conducted rituals and living sacrifices to reawaken what they believed was a loa. Awakening at these events and promptly slaying the trolls, Kith’ix made contact with the dormant aqir, who had remained hidden underground since the fall of the Black Empire He ordered them to begin to build up their numbers, and they created a vast subterranean empire called Azj’Aqir. Finally prepared, the C’Thrax drove the aqir into a frenzy and urged them to lay waste to the bustling troll civilizations, with the goal of pleasing the Old Gods and to begin cleansing the world for their eventual return. The aqir swarmed the troll lands and began systematically destroying the majority of smaller tribes.
The land you’re referring to is land the Aqir retreated to in order to launch another attack, not the Aqir’s indigenous underground land.