Yes, they heavily rely on AI now. Their art had a sudden change in style to this very clear StableDiffusion generic lighting/shading and proportions. You can see the sudden jump in their Twitter gallery, right at the same time AI started being able to output something reasonable. You can even see a lot of the weird artifacts in their works during that transition when they started trying to use it...
Before AI, Oct 2022: https://xcancel.com/KurehaTateishi/status/1581200771034779649
Mar 2023, started experimenting with it, shading style is like nothing they've ever done, but you can see it clearly struggled with the skirt:
Like c'mon now, bold doing such a generic StableDiffusion type image lmao: https://xcancel.com/KurehaTateishi/status/1847523742383235109 https://xcancel.com/KurehaTateishi/status/1726522568109351106 https://xcancel.com/KurehaTateishi/status/1878284593075495296
That's not to say it's entirely generated, but they obviously are generating something, drawing their own lines to make it look more like the character they want and then feeding the flat shaded image back through AI to shade it proper again. But they aren't covering their tracks enough for it not to trigger HIVE detection on many of them.