Although according to the strength of that Divine City, the number of Heavenly Gods within is surely far more than that of Zhou Tian's team. However, due to the vast area of the Divine City, ensuring that no internal problems arise within it means that the majority of the Heavenly Gods must be dispersed to guard key areas.
Thus, although they could mobilize quite a number of deities to deal with Zhou Tian and his group, for various reasons, the amount of strength that the Divine City could marshal at that time was limited. As long as Zhou Tian's Heavenly Gods avoided being surrounded, it wouldn't be easy for those Heavenly Gods to detain them.
The only concern for Zhou Tian's Heavenly Gods was if the true gods of the Divine City decided to take action, given the strength of a true god, it would not be easy for Zhou Tian's group to resist should they indeed intervene.
