No, it's not necessarily constructed from real CSAM. It can be constructed from real photos of children and real porn.
Anyway, when saying that someone should be jailed, you should consider all the main possibilities.
EDIT: And when you have real porn with adults, hentai with supposed adults and anime with children, that might be good enough to generate something that's technically like generated CSAM, even if the quality might not be too good. Anyway, I'm not very well-versed on quality of CSAM.