I agree that arbitrary limits can be a bit restrictive at times, but that is mostly an issue when dealing with automated code checking tools.

However, in this case it is more about something to keep an eye for for the reviewer -- if the function is large and complicated it may be a good idea to take a closer look at it. Presumably the reviewer won't force the function to be broken into smaller pieces unless it actually improves readability and maintainability of the code.