To understand that, we need to go back to the Soul Binding Contract itself. The contract stated that the cultivators couldn't talk about the sect and its secrets 'unless they were given permission to do so.' One of the permissions that the sect members had was to recruit more people! That's how this many extra cultivators suddenly appeared when the time came to leave Glinyer.
As for the reason why, it was basically the average strength of the System Sect's members. Quite a few of the cultivators of the sect could now fight one stage or maybe even two stages higher than average cultivators at their level.
That made sense since all the cultivation techniques in the sect were at the level of a continent's ruling power. Well, at least a continent similar to Huring and the others, not the powerful ones in the center of the Realm of Gods. In any case, combined with Roan's training and the great cultivation environment, everyone grew much stronger.
