I think what the author's getting at is that the function you pass to JavaScript .sort() just returns a number — not special sentinel values — which represents the relative order of two items in the array. By calling that function strategically from a loop, .sort() can sort the array.

Similarly, an SDF returns a number that represents how far a point in 3D space is from an object's surface. And, by calling that function strategically from a loop, you can draw a 3D scene using only that function.