So technically speaking the % dark is the amount of cocoa (solids+butter) / total. There really isn't a standard way that dark chocolate is defined otherwise.
Therefore you can have a 75% dark chocolate that is well balanced (70% cocoa, 5% added cocoa butter, 25% sugar) but have it be less dark than a white chocolate that is 80% cocoa butter and 20% other stuff since it's technically 80% dark.
Not really sure where I'm going with this other than to say generalities in chocolate, like saying all dark chocolate is sour, suck. In any case, white chocolate is usually crap
They're saying that the % in dark chocolate is percentage of cocoa in general, which would include both cocoa butter (the only "chocolate" part of white chocolate) and cocoa solids (the fermented product of the cocoa bean, what I would consider to be chocolate). I'm not sure how accurate this is because I always thought it was a measure of percentage of cocoa solids, not just "cocoa" in general which would include the butter. I could be mistaken.