Wasn't the congregational Qiyam al-Layl mentioned in the evidence without prior arrangement, and wouldn't arranging it on a specific day be contrary to the Sunnah?

In summary, making appointments to observe a specific night [in worship] is not prohibited unless it becomes a regular habit, because the forbidden act is to liken something not legislated (non-Sunnah) to something legislated (Sunnah). However, prior arrangement with the intention of motivating, invigorating, and reminding people of the sunnah of night prayer is permissible, as long as it does not become a regular practice, thereby resembling a continuous Sunnah. It is permissible to gather for acts of obedience such as supplication, recitation, or standing [in prayer], as long as this does not become a continuous practice, thereby resembling something legislated. Thus, there is a difference between what is adopted as a regular habit and what is not.