Bresenham's algorithm (which was invented around 1960 to draw straight lines at arbitrary angles on pixellated displays) was groundbreaking because it didn't require any multiply or add operations.
Sigma-Delta is basically the same idea as Bresenham's algorithm in a different kind of space.
In effect, Bresenham ends up performing a division by repeated subtraction. And at every subtraction step, it outputs a pixel.
Imagine if you did dithering to produce grayscale images with a black and white screen by just using different sizes of rectangles; you'd have to have a very high resolution screen before you wouldn't be able to see the individual rectangles. That's what pulse width modulation is like. Whereas pulse density modulation, using a sigma-delta modulator, is more like a proper dithering algorithm, where you spread the pixels out evenly across space, you don't just use different size rectangles to represent different shades of gray.