No such assumption is made in the article.
Nor does it give a single answer.
Mere prompting is still not enough for copyright, and the problem is unsolved on how much contribution a human needs to make to the generated code.
In the case for generated images copyright has been assigned only to the human-modified parts.
Even worse, it will be slightly different in other nations.
The only one that accepts copyright for the unchanged output of a prompt is China.
Here's a question I have: if the AI generated image is of a character of which you own the IP, don't you have protections based on the character regardless of who gets copyright protections from authorship of the image?
Yeah if you have a copyright on the character, the AI generated image doesn’t change that. It doesn’t give you more of less protection than you already had.
IANAL but this sounds more like trademark territory.
You can also trademark a character if it’s used as a brand identifier in commerce.
There are far more characters protected by copyright than trademark.