It’s not child sexual assault if there was no abuse. However, the legal definition of csam is any visual depiction, including computer or computer-generated images of sexually explicit conduct, where […]— (A) the production of such visual depiction involves the use of a minor engaging in sexually explicit conduct; (B) such visual depiction is a digital image, computer image, or computer-generated image that is, or is indistinguishable from, that of a minor engaging in sexually explicit conduct; or © such visual depiction has been created, adapted, or modified to appear that an identifiable minor is engaging in sexually explicit conduct.

You may not agree with that definition, but even simulated images that look like kids engaging in sexual activity meet the threshold for CSAM.