There's a saying that one mountain cannot contain two tigers, and the sea already has a Sea God. From the very title of Sea God, one can discern their ambition; they would never tolerate the existence of a Serpent god to compete for faith in the ocean. These two demigods are inherently adversaries.
By drawing the army from Yuyuan City to attack the Black Water Swamp, Lin Xiao intended to set these two forces against each other, stoking the flames from the sidelines until the opportunity arose to break through the defenses of both the Frog People and the Snake Tribe, shifting the Snake People Demigod's rage onto Yuyuan City and making his escape.
The plan was sound and feasible, but whether it could be successfully executed would depend on real-time maneuvering.
Moreover, before embarking on this mad scheme, he needed to complete another task of slaying a Sea Beast first and secure the reward.