I would put it differently. At 30 bugs per kLOC, I'd prefer my codebase expresses a problem & it's solution- and as little below that level as possible.
Each well-vetted layer of abstraction between a scientific programmer and the machine's low level interface eliminates whole classes of bugs that are irrelevant to the problem that user is actually working on.
Where does that number come from?
The important part wasn’t the exact number, but rather the discovery that the ratio is pretty stable.
The thing is, it isn't stable. It just doesn't depend on the language, what is very surprising. But it varies enormously from one study to another and, AFAIK, nobody has a good set of factors explaining it.
At that constant level of effort, we make a certain constant number of mistakes. And that's what I think these studies show.
Personally, I suspect the number of bugs per line is defined by social and psychological factors, and what changes from one language to the other is the amount of effort one has to put into testing and debugging. But well, none of this is obvious to me.