Unless AI companies already developed and launched plugins/extensions for people to do something that looks like hand drawn sketches inside of Clip Studio, and suddenly got a lot better at understanding prompts (including having inspiration of their own), then I'm pretty sure it's a human.
I don't think I'd get to see in-progress sketches and it wouldn't be as good at understanding what I wanted to have had changes then. I've used various generative AI image generators (latest one Qwen Image 2511 and a whole bunch of others) and none of them, including with "prompt enhancements" can take very vague descriptions of "I want it to feel like X" or "I'm not sure about Y but something like Z" and turn it into something that looks acceptable. At least not yet.
And because I've spent a lot of time with various generative image making processes and models, I'm fairly confident I'd recognize if that was what was happening.
These are very important questions that cut to the heart of "what is art".