Looks like it's built just to sell you a subscription to their AI. On that note, it doesn't look particularly geared toward fiction either.
I bit the bullet and created an account just to test it out a bit more.
$12/mo annually, $18/mo otherwise. (Edit: It seems free to use, with limitations on number of AI checks you can run and the model complexity on the free plan)
All of the grammar checks are AI. There are no non-AI grammar checks. I don't know that I trust AI to actually give correct grammar suggestions at this point.
You can select from multiple AI models if you have one you like already ... but then, you could use a different writing software and subscribe to those services directly, most likely.
I suspect it uses Markdown behind the scenes, given it's very limited in formatting options. E.g. you cannot choose text alignment or other text formatting options. That might be fine for some, but it's annoying to me because I can't do some formatting, e.g. small caps. You can at least change the font the entire document is rendered with, so if you prefer a serif font you have that -- but it's a web app and limited to a small selection.
I don't know. I would simply recommend anyone interested in this just download Obsidian.md and subscribe to their favorite AI of choice.