Sure, but there's a lot more to the example that a single component. There's the app component, 3 other components, a reducer, and a custom hook. Putting all that in a single file is fine for an extended example explaining how things work together with a further chapter explaining how to break it down in to separate files, but it's not how you'd write a real app. The article author clearly doesn't know that if his criticism is that Ember does it better by using more files, so it not unreasonable to question if his knowledge of React is up to comparing it to Ember. It's quite a fundamental mistake he's making.
