It seems to be mentioned, unless I misunderstand what you mean. The second-last section in the article begins with this:
> If you really need to do some CPU intensive work, it’s now possible to offload some of the workload to a language server. This allows you to implement the bulk of your extension in another language (for instance, writing Rust code and compiling it down to WASM).
I think you're right. I got confused because he talks about compiling to WASM; I'm not knowledgeable about WASM but a language server is an entirely separate process, which means you can compile your Rust to native x86 or whatever you want. I assume Rust compiled to native machine code is faster than WASM? I guess the one advantage of WASM in that case is that you don't have to compile the language server for every platform the extension runs on. (In my case, I compile the language server for x86 Linux and Windows.)
> If you really need to do some CPU intensive work, it’s now possible to offload some of the workload to a language server. This allows you to implement the bulk of your extension in another language (for instance, writing Rust code and compiling it down to WASM).
The section above also mentions tree-sitter.