The willingness to open up Ai Zhen Zi's Celestial Domain and invite Qing Emperor Alliance players over indicates a lack of manpower, but ultimately it seems to be a form of compensation for previous missteps.
Even despite the intense hardships and heavy casualties in the development of Xu Ling Celestial Domain by the Shushan Sword Alliance, they never asked for assistance from other factions, securely keeping all benefits for themselves.
Survivor of Hundred Battles lived up to his status as a leading figure by offering such conditions. Even if Ye Rong was reluctant to readily forgive the past transgressions, both Yu Shuangxue and Wind Light and Cloud Faint, who had suffered ambushes, would actively ask him to accept the terms.