NONREQUIRED ELEMENT OF OFFENSE.-It is not a required element of any offense under this section that the minor depicted actually exist
That section of the US code covers obscene visual depictions of minors. Different from actual child porn but punishable with the same statutory minimums/maximums. I could only find a handful of (publicized) actual prosecutions that used that law. Here is a notable one: https://www.justice.gov/opa/pr/texas-man-sentenced-40-years-...
In that case, he was obviously a sick person and running a commercial enterprise that catered to pedophiles but none of his material involved actual children. They were short stories and cartoons. He appealed and lost.
Another one where a registered sex offender downloaded some hentai. He was sentenced to 20 years and also lost on appeal: https://law.justia.com/cases/federal/appellate-courts/ca4/06...