I'm sure that nvim-lspconfig doesn't take care of installing binaries for LSP servers like this. You either set the path to the binary in your config, or you are using another neovom plugin to make sure LSP server binaries are installed
No you're right it was mason, which installs lsp servers to a location it manages. I had set it up once and then forgotten. The complexity of all the interacting packages needed with neovim does add to helix's appeal.
From a brief experiment basic lsp usage seems to work in helix without any config changes, so perhaps that's changed since you set it up.