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.