Actually 8x cards are not designed for 1.5v, but 0.8 volt. 1.5v slots are intended for the 4x agp 2.0 spec. Most 8x cards are supposed to be backwards compatible with a 4x 1.5v slot, but that doesn't necessarily mean that it will run stable or at all for that matter. If your agp slot is 3.0 compliant it will run 4x or 8x cards.
As I stated it is supposed to be backwards compatible. According to the link you provided it states pretty much the same thing as I said previously.
"a motherboard that supports 1.5V (AGP 4x) video cards should work ok with a .8V video card (AGP 8x), however in the later example the video card would only work at the fastest speed the motherboard supports (AGP 4x)."
Should and always are different things. AGP8x cards always work properly at 1.5v because, as it turns out, every single 8x card on the market it 4x compatible. Over half of them are even 2x compatible.