Forcing somebody to write short sentences wouldn't improve his writing.
That said, a good writer will know when using a long sentence is the right choice for emphasis. And there's no way this program could ever develop that level of good judgement. This program isn't good for writers. It's good for non writers who want to quickly improve their shitty writing without effort.
See, I don't think the above is any better as two sentences, and I think it works better as one. Perhaps I just don't like starting sentences with "So".
Good writing is sensitive to multiple contexts and multiple rules. Essentially, one has to be able to shift-gears and notice the results of your writing, how your audience will receive it.
The only way a person becomes a good writer is through self-direct study. A bad writer who writes long sentences without noticing the results will remain a bad writer if someone forces them to divide their sentences, because they are still not being aware of the results of their writing.
All that said, a lot of people learning to write create long sentences because they don't yet have the skill to communicate with shorter sentences. Demanding that a person break up their sentences into smaller chunks can help them become more skillful - if they put in the effort to learn this.
Showing someone overly long and complex sentences, and pointing out examples of passive voice could only improve an editing process. Once you manually point out problems, the burden to change/leave as is falls onto the writer.