The complete advice should be "Write short sentences unless you are a good writer."
Most people are not good writers (that includes me), and good writers don't need this generic low level advice, so "Write short sentences." is fine. Also, it is more what you’d call a ‘guideline’ than an actual rule.
Most people are not good writers (that includes me), and good writers don't need this generic low level advice, so "Write short sentences." is fine. Also, it is more what you’d call a ‘guideline’ than an actual rule.