Definition of "bug".....or definition of "defect"?
I'm not sure what the paper is trying to measure, since there is a hierarchy of defect types with the most immediate defect being "fails to compile" or "fails to run"........all the way up to "fails to match the original business requirement". In this latter case, the code might be technically perfect!
Examining Table 5 in the original paper ("Categories of bugs...") it would appear that failure to match the original business requirement is NOT considered a bug!!!! So perhaps the rather technical focus of the paper misses the main point of having software in the first place.