I would much rather check my writing against grammatical rules that are hard coded in an open source program—meaning that I can change them—than ones that I imagine would be subject to prompt fiddling or worse; implicitly hard coded in a tangle of training data that the LLM would draw from.
The Neovim configuration for the LSP looks neat: https://writewithharper.com/docs/integrations/neovim
The whole thing seems cool. Automattic should mention this on their homepage. Tools like this are the future of something.