This quote is so preposterous it almost made me want to stop reading the article. Different types of writing should have different sentence lengths. I'm quite sure the sentence length for a math textbook and a Clorox ad should be different. There's a reason it's not common practice to measure one's average sentence lengths.
Like most material written on language, this article says almost nothing and is basically filler. "Don't be afraid to give instructions". "Use lists where appropriate". Well of course we know we can give instructions! I recently read a famous book called "How to Read a Book" that in a similar vein struggled to find anything non-obvious to say about language. It just comes so naturally to people that it's difficult to comment on. This article is coming from an organization called the Plain English Campaign that's been around since 1979. I wonder what they can have claimed to have accomplished since then.
This article reminded me of arguments I always read in philosophy about tough subjects like radical skepticism where the authors fail to make any points beyond the obvious. Sure, certain knowledge is impossible for humans to attain, but can you say anything else!?