> It's usually the opposite. And the post is specifically about making JavaScript tools, why would you not expect them to be written in JS?
Take a look at rollup, vite, etc. These tools are essentially replacing webpack, which is written in JS. Modern Rollup (^4) uses SWC (Rust-based bundler), and vite is currently using a mix of esbuild (Go) and Rollup. I think they're switching to SWC in v6 though.
The point here is that for certain operations JS is not nearly as fast as lower-level languages like the aforementioned. Bundling is one of those performance-critical areas where every second counts.
That said, as a TypeScript developer I agree with the sentiment that JS tools should be written in JS, but this isn't a hard and fast rule. Sometimes performance matters more. I think the reasonable approach is to prefer JS – or TS, same difference – for writing JS tools. If that doesn't work, reach for something with more performance like Rust, Go, or C++. So far I've only had to do the latter for 2 use cases, one of which is hardware profiling.