When Linsley used his Annihilation gift, the entire Outer God's dimension was destroyed in a single blink of an eye!
Even the remnants of the Outer God's power were obliterated, leaving absolutely nothing behind, as if the Outer God's dimension had never existed at all.
Since he had destroyed the Outer God's dimension, he returned to the real world.
As he returned, Linsley spread his senses throughout Terra and noticed that the Outer God's lackeys were still actively engaged in battle with the magical girls and mecha drivers, seemingly unaware of the Outer God's death.
Usually, killing an Outer God wouldn't truly kill them unless all of their lackeys were also defeated. After all, the Outer God would leave part of his will within his followers, allowing him to resurrect through them.