LSP server does not have to implement all of the spec to be useful. Just using document change and diagnostics allows to implement basic linter or spell checker.
Yep, the LSP mentioned in the blog post (I'm the author) only implements diagnostics and autoformatting for now. With just that you can already provide a lot of utility.
This is my starting point for implementing language servers in Zig: https://github.com/kristoff-it/zig-lsp-kit