They copied literally a handful of lines of code that an intern could have written in a few hours.

But they didn't just write their own APIs. They copied them. Thousands of them.

Copying an API is what software developers call "implementing an API"

When you create your own USB cable, or electric plug, you aren't copying the API specs, you're implementing them.

Once they admit to copying some of it, it throws the whole clean room process into doubt. I think the courts are working on the "where there's smoke there's fire" principle.

IYO did they copy an artistic work?

Copyright covers subjective work, not just "artistic" work. In the same sense that the choice of how to paginate a phone book and where to put the guidewords on each page is copywritable, yes APIs are copyrightable.