The point of the article is that you shouldn’t write only short sentences, not that all sentences should be long. Furthermore, there is a full paragraph made entirely of short sentences to prove a point; taking an average from the whole text is skewed information.
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.
Becoming a "good writer", like becoming a good anything, is a matter of practice and study, of learning to use the tools which language provides. The ability to construct well-formed long sentences is one such tool. It may take time and effort before you can use it effectively, but this is the case with most tools which are worth using. To reject this tool because you are afraid you may use it badly is to needlessly limit what you are able to do.
This might be a worthwhile trade if you really could improve your text by only using short sentences, but sadly, you cannot. The central unit of meaning in written text is not the sentence, it is the paragraph. If you take a sentence that is long, discursive, incoherent and ill-formed, and break it into short sentences, this will not magically resolve all other things that are wrong with it. Fixing the text entails organising its elements so that they develop logically. If they are entertaining, rhythmically varied or otherwise engaging to read, so much the better, but the primary challenge is to organise what is said. If you can do that when it comes to the relationship between your sentences, then it's not any harder to do it with the clauses in a longer sentence. And if you can't, your prose will still be bad.
[0] https://techcomm.nz/Story?Action=View&Story_id=106