Yeah, a "15-20% lie" means it's 15-20% worse than usual.
As you rightly say, this is ambiguous and could just as well have been meant the other way around: that a "15-20% lie" would be 80-85% worse than usual. (This is, in fact, how TWO indicates lies. So a "15-20% lie" in WGT corresponds to a "80-85% lie" in TWO.)