Her confidence comes not only from herself, but also from Sif.
The power of Sif back then was far less than hers now, yet she managed to break free from the Netherworld and charge straight toward the Sea of Annihilation, which clearly demonstrates the true strength of the Netherworld.
If they truly had strength, they wouldn't be confined to the prison of the Land of Death.
The Gate of the Netherworld is right there, yet they don't even have the courage to leave. Clearly, they are not even a match for the Netherworld's army.
After all, the Netherworld is backed by the Death God, and controlled by the Nether Empress Judica at the Nether River Shore, where countless beings from the Material Plane enter the Netherworld as soul bodies, becoming resources for the Netherworld.
The slots for reincarnation are limited, where can those soul bodies go that cannot reincarnate? Naturally, they turn themselves into bricks to build the Netherworld.
Sofia is combative, but not reckless.
