It's a nice aphorism, but it's a very bad analogy. Before the build of an aircraft is started, the target weight of the assembled product is (roughly) known. Therefore, in the process of building an aircraft, the assembled weight at least goes up from 0 to $target -- in an erratic fashion, but it's still a continuous measurement.

I have never seen a program design specification that even attempted to quantify the lines of code of the end product, let alone be accurate.