I think the rule that "functions should be small enough that they should be easily reasoned about" is a reasonable rule, and it makes sense to follow it 95% of the time.
"Functions should be 5 lines or less" is a measure that approximates that rule, but isn't exactly the same thing - I hope you agree we could both come up with 4 line functions that are impossibly complex or 6 line functions that are easily reasoned about.
I think with Clean Code (and a lot of these kinds of things - Design Patterns is a great old example of this), people can get too dogmatic about applying these sort of approximated rules, when it would make a lot more sense for someone else (i.e. not the code writer) to use their best judgement and just directly answer the question "is this function easy to reason about?" rather than using the approximate measure.
> "Functions should be 5 lines or less"
This can’t really be a serious guideline unless you are writing APL, in which 5 lines can already be daunting to grasp. This guidance depends on the language. For Python, once you get over 50 lines it starts to look like you don’t actually know what you are doing anymore.