PNG-8 is different from PNG-24. PNG-24 is the file format for alpha transparency which IE6 can't display. PNG-8 uses "binary" transparency similar to a GIF and does work in IE6.
PNG-24 allows you to create semi-transparent backgrounds (like a 10% to 50% opacity gradient), PNG-8 does not. The image you linked to above does not need a semi-transparent background.