I thought they meant as opposed to, say, emacs where the LSP client has to be installed as a 3rd-party package (eglot or lsp-mode). As you say, the server is in a different process by definition of LSP. I don't know vim but from skimming the article it sounded like, until neovim 0.5, for vim one also had to install the LSP client as a plugin. In VSCode in contrast, the LSP client is native in the sense that it is built into VSCode. That makes a big difference from Emacs at least, because the LSP clients (eglot at least) are still under heavy development.

Means you don’t need a plugin. Which is apparently a big deal to some people. Maybe the don’t know how to install plugins...