I think the fact that this doesn't evaluate your prose is a major plus. Grammarly is OK if you don't speak English very well and need someone to tell you what to do. But these are exercises for fluent writers to get better, and they are not going to want their writing evaluated by a bot.
> What do you think is the applicability of this to fiction writing?
Possibly. Knowing how to be terse and how to be meandering is a valuable skill in itself. In the business world, a long winding approach can be helpful to take the spotlight off of something. In the fiction world, maybe you could use long-winded dialogue to establish a particular personality for a character.