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?