I start with the very debatable axiom that all new code begins its life as liability. I've argued this in the past by deduction:
You type a single character into your editor, and (depending on the change, and your language's execution story), you've probably broken your code. Type a few more characters, and it is now less of a liability, because it compiles. A few words more, and it is less of a liability, because it accomplishes some small task (imperfectly). By the time you have committed some set of changes into version control, you've hopefully ironed out the liabilities. But, it's entirely possible that you've created more liabilities than assets. Code review hopefully refines it further, but things still sneak through. I could keep going, but I think this is clear?
Measuring the liability:asset ratio for a given piece of code is a challenge, and is more qualitative than quantitative. I believe that this is also an argument in favor of "new code is a liability", otherwise code review should catch all of our mistakes.
But yeah, I have found it to be useful "in the real world" as a heuristic for thinking about how code bases evolve over time. I do not think that it translates into a hard-and-fast rule to weaponize in code reviews. However, it does temper your expectations about the fallibility of code, and it is also a convincing argument for code review as a quality-control process.