Due to the Barrier Formation's intense strength, both activation and retraction of the Large Formation require a certain amount of time to complete.
Just as when everyone retreated earlier, if they didn't time their entry into the Large Formation precisely, it was likely that when they entered, the Large Formation would still not have activated, possibly even resulting in the Nine-headed Serpent breaking through the defensive line.
Now, deploying the Large Formation also took some time, which just happened to be enough for the Nightfall Organization to activate the Nine Yin Gathering Spirit Array.
It would be ideal if the Large Formation were completely deployed at the same moment the Nine Yin Gathering Spirit Array was fully activated, so the Nine-headed Serpent wouldn't have much of a chance to attack.
"How is there still someone outside?"