I think because Claude said so and the author just copied it without checking. You can see it in the first screenshot of Claude's output:

> After examining the provided code, I've determined that this appears to be a CLI application for Claude code-related functionality, built as a CommonJS TypeScript application that has been compiled with webpack.

Although, looking at the minified code, it seems to be using module.createRequire for CommonJS compatibility, so maybe it isn't completely wrong: https://nodejs.org/api/module.html#modulecreaterequirefilena...

.mjs typically designates newer ECMAScript modules, contrasted with .js for older CommonJS.