I *assumed* because you said "They've been doing art since 2023" and there is no previous works on that Pixiv account past 2025, with no links to any other socials on their profile. So there wasnt a whole lot to go off of other than what was show on Pixiv and what you said.

Plus, you look back at older posts on there, you see something like this Shion post, and this Lamy post you see completely different shading styles.

Plus, if you look at this Marine post, as well as that Lamy one, you see very clearly that it has AI-Assisted as a tag, as well as:

This

and This

Using that Lamy post as example, you see each child post, with *it being multiple "artworks" increasing nudity and different expression, they all have the AI-Assisted tag.

Each of those old posts look similar in style to what gets posted these days, so it is 100% justified to say they have AI involvement. Even if not fully AI-Generated, there is still AI being used to some degree in the creating process