GPT-3 is constructing entire narratives from prompts. It operates at the level of multiple sentences and paragraphs, to the scope of an entire essay itself. Based on a provided prompt, it generates an essay.
By your same logic, an instructor assigning a class essay would be considered the author of those essays as that instructor had generated the prompt.
Moreover, as a matter of pedagogy, where a course either assigns group work (such that multiple students are contributing to an essay), or requires drafts and reviews with an instructor, TA, or other students as part of the overall essay-writing assignment, those become acknowledge parts of the process. In the case of the draft / review / revise cycle, it would seem that that is one way of putting a check on AI-generated essays, at least for now, as though generators can follow prompts, they're rather more limited in taking criticism well, and adapting to it.
Hol up, didn't you just describe academic research?! Sounds like we're on the right path here.