This is a very strange nitpick. I don't think many people at all would consider the Arduino libraries to be an OS. The UI, apps, usage of the dual screen, keybo

This is a very strange nitpick. I don't think many people at all would consider the Arduino libraries to be an OS. The UI, apps, usage of the dual screen, keyboard, and scroll pad are more than enough to make calling it a custom OS reasonable. Just because they didn't write their own filesystem and related tooling.