Can't help but notice that this line - "If you see a red highlight, your sentence is so dense and complicated that your readers will get lost trying to follow its meandering, splitting logic — try editing this sentence to remove the red." is shown as red. I don't understand why this is a complicated sentence. By forcing the writer to split a good line into smaller ones just because some reader might not be able to follow it is wrong.
Another sentence I tried - "I wasn't that interested in working on my project today and hence I preferred to stay at home and watch Netflix in spite of the multitude of issues reported." is show as very hard! It makes me question if the original developer of this app has even read good well-known style guides for the English language.
I'd advice you to read a good style manual to understand more about writing and how to use words. One doesn't have to forcibly introduce words that are too complicated. There are good organic ways to write sentences that employ seemingly complex words in a natural way so that readers can understand quite well. The goal should be to elevate the users to get better at reading in addition to conveying your material, IMO.
One doesn't have to degrade their sentences just because we live in the age of attention span deterioration and social media. All of these minor tweaks can help people improve their attention spans by releasing micro dopamine orgasms.
http://www.hemingwayapp.com/
I write/edit blog posts for a living and that's always my first piece of advice.