Even though Sterl knew how powerful the legendary army was when combined with the formation, he still pushed forward with the hundred Celestial Stars for a reason.
The reason lay in Boris, who was a dark piece in this game. As the overseer of this formation, Boris controlled most of the powers within it. With just a subtle manipulation, he could temporarily disable the formation's effectiveness. If done correctly, it would go completely unnoticed.
Because a formation is still a formation, there's always room for error. Sometimes, when too much energy pours in, or there's a slight pause in energy transmission, small faults could appear.
Even so, as long as the formation was maintained, it would automatically generate a defensive barrier. Although it wasn't as strong as a deliberately built one, it would still be enough to block the attacks of an ordinary god.