So, it wasn't that the group of calm demigods were unaware of how disadvantageous the current changes were to them; it was rather that at that time, they had no other choice. In a situation where no other method was available, the demigods were forced to react this way.
After allowing a portion of the demigods to withdraw, the ones who stayed behind knew full well that even if they joined forces, they were no match for Zhou Tian. Knowing that retreat was akin to a dead end, they had no other options at the time and could only gamble that Zhou Tian would not attack their Ancient Ruins Space.