I think they're claiming a lot more than "here's how to write concise sentences, if that's the style you're aiming for."
Note also that this section ("Clear, Concise, Sentences" [sic]) falls under the heading "Improving Your Writing Style."
Well I don't see the problem here. It is true that many righting concise sentences can help many people improve their writing style (it's the case for foreigners like me).
I think the site targets two types of audience:
- Those with very poor style who want to improve
- People with good style who just want to learn how to write concise sentences.
I'll give an example illustrating the second point. For some of my school projects, I had to review a lot of "science project reports", and similar stuff. Well trust me, it is painful to read a report with long, convoluted sentences expressing a simple idea..
Good writing skills are so important today when you consider that much of the writing people typically do is in an email or a powerpoint, not a romantic novel.
To look at it another way, these are the rules that are made to be broken in the service of stylized writing. If you don't know the rules, you won't know when to break them.