> [Vector format] means that you only need one file per icon; it doesn’t matter how many sizes you want to render icons at.

That's not really true, even with vectors it's good to have different images for different sizes, that add details (for larger sizes) or remove details and increase component sizes (for smaller sizes), so the icons look good at different sizes. You can see in the tape recorder icon the author shows: at larger sizes, it looks much better than the bitmap images, but at smaller sizes the bitmapped icons look better; the vector icons become a mush of unclear elements. The bitmap shows pieces as proportionally larger than they should be, so the visual elements that you consider important still show clearly.