They're likely referring to Babel and TypeScript. In short, it doesn't matter what you're writing as long as your transpiler has the right `target` so it works in older browsers/engines too.
There are popular JavaScript tools that nearly everyone uses, that reimplement Lisp poorly by source-to-source compiling stuff written against the latest new hotness spec into something that can be run by some old-and-busted JavaScript runtime (e.g., in browsers).