Maybe the useful test is not “who wrote this line?” but “can you show how it went from requirement/prompt/context to diff to human review/tests?” If you can’t, ownership is only one issue. You also can’t tell what was accepted as engineering work versus just copied output.
This is actually closer to how the Copyright Office thinks about it than the article makes clear. The registration guidance that emerged from the Thaler proceedings specifically asks applicants to describe the human creative contributions and how the AI was used. A documented workflow showing requirement, architectural decision, rejection of AI output, human restructuring, and review creates a paper trail that maps directly onto what the Office looks for. The can you show how it got here test you are describing is the practical version of the legal standard.