At least at the time, the majority of demigods besieging Zhou Tian thought that although Zhou Tian's displayed strength seemed to be a bit superior to theirs, they held a numeric advantage. Thus, even if those forces didn't unite then, should they truly have the will to fight Zhou Tian, leveraging their superior numbers would make winning not an impossible feat.
Moreover, even when Zhou Tian exhibited part of his strength, there were still demigods who believed they could win. That's why those demigods were still considering battling Zhou Tian, not fleeing in fear because of his performance.
Zhou Tian was naturally pleased to see things progress in such a way, as those weaker demigods weren't of much value to him. Even if used as Zhou Tian's practice targets, he found their strength lacking. So, if they were willing to leave under those circumstances, given their limited utilitarian value, Zhou Tian would naturally let them go.
