That is actually a really bad rule, though you probably are only joking.
No, I wasn't.
There are many examples of short, but very valuable code. Just think about anything math or physics related.
A rule of thumb is not a strict law. I never disputed that there are certain edge cases. What has to be considered but is not on the radar of most people: Threshold of originality. A "valuable" 3 LOC bash script is likely not being able to be copyrighted in the first place. In cases where the work is tedious but not creative, the work may also not be able to be copyrighted (depending on jurisdiction). See https://en.wikipedia.org/wiki/Sweat_of_the_brow whether a certain jurisdiction protects tedious work or not.