Proselint isn't a grammar checker. It's a "style" checker, warning writers when their work is hackneyed, inconsistent or very obviously not great.
proselint places the world's greatest writers and editors by your side, where they whisper suggestions on how to improve your prose. You'll be guided by advice inspired by Bryan Garner, David Foster Wallace, Chuck Palahniuk, Steve Pinker, Mary Norris, Mark Twain, Elmore Leonard, George Orwell, Matthew Butterick, William Strunk, E.B. White, Philip Corbett, Ernest Gowers, and the editorial staff of the world's finest literary magazines and newspapers, among others. Our goal is to aggregate knowledge about best practices in writing and to make that knowledge immediately accessible to all authors in the form of a linter for prose.
It's in rudimentary form at the moment, but expect it to turn up in web-based form fields and popular apps soon. See also Hemingway App, which does a similar thing but with an eye toward concision and brevity rather than general style.