I should have addressed the original article. As a sibling commenter mentioned, I didn't say all flat designs are good, quite the opposite. That to-do list design is very mediocre.

A desktop to-do list app that I think is super well done is Reminders on MacOS. Its basic functionality is immediately intuitive, and it gives love to power users too with its secret ability to parse natural English (e.g. "laundry at 9") for adding notification times.