This is built on so many bad assumptions. At best the "rules" it's trying to enforce are training-wheel rules, the sorts of rules given to novice writers to help them avoid flabby, purple writing.
But the assumption that short sentences are better than long sentences, or that simple sentences are better than complex sentences is just wrong. There are all kinds of reasons why you might use one type of sentence over the other or vary them for effect. You might be concerned about rhythm, or you might be attempting to establish a certain tone, distance, closeness, formality, or lack of.
We have this weird cultural obsession with the clarity, brevity, and simpleness of writing. Jacques Barzun even wrote a writing manual called Simple and Direct, as if these are the only virtues to be found in writing.
But I think you want as many tools as possible to achieve the effects you want. There is a huge rich tradition here, that we've largely lost, a tradition that teaches about hypotactic and paratactic sentences, that teaches about periodic and loose sentences, that teaches how to make left and right branching sentences, that teaches subordination, that teaches rhetorical devices, and that advocates (at times) longer, more complex sentences for richer and denser writing.
Thankfully there are a number of books out (some of them) recently that seem to be fighting back against the austerity view of writing.
They include, if you're interested:
- Brooks Landon, Building Great Sentences
- Stanley Fish, How to Write a Sentence
- Virginia Tufte, Artful Sentences, Syntax as Style
- Richard Lanham, Analyzing Prose
I'd just add, there is nothing wrong with being simple and clear. There is nothing wrong with cutting out needless or weak adverbs. But there is something wrong with worshiping the austerity style as, at all times, the best and the only way to go. There are lots and lots of reasons and occasions to deviate from it, but the style orthodoxy these days is the one assumed by that (admittedly cool) website.