The point of the analogy is that both metrics are, fundamentally, useless. A 787 (unloaded) weighs about 175 tons. Does this mean that putting 175 tons of material at the end of the assembly line corresponds to the complete and proper assembly of a 787? Nope. There are other far more useful measures: does it look right, is it fitted correctly, does it pass QA (testing), is it composed of the correct pieces or just things that resemble them?

In software, LOC is, itself, a useless measure for progress for the same reason: It indicates nothing of value. I have inherited programs in the 10s and 100s of thousands of lines of code that I was able to reduce to less than 20% of their initial size without loss of capability, and generally with improved performance, stability, and maintainability. The right measures are around functionality and the other meaningful qualities (stability, security, testability, etc.).