The Euclidean sphere-volume formula? A black hole doesn't have a spatial volume that's defined in the absolute sense (unlike a surface area); you can take a spacelike three-dimensional slice of its interior and get almost anything. That's on top of the difficulty that the Schwarzschild radial direction is timelike inside the horion, while Schwarzschild time is spacelike, which is why integrating its area over its lifetime can be handwaved as in some sense more 'reasonable', say for a distant stationary observer.
Though your main point stands completely unaffected.
