No, the lj_biz post linked to a completely different law (one for actual or indistinguishable, as you say).
The original fictional child porn was struck down by the Supreme Court in 2002. Since then, the Government has brought in a law (PROTECT Act 2003) which criminalises fictional child porn, but only if the images are also considered obscene.
. Apparentely only one person has been convicted under the law - and that was a case involving depictions of prepubescent children, and the guy also possessed actual child porn.
That's the law the NCMEC quoted.
Anyhow, whether the law says these images are illegal is beside the point. The point is that LJ said these images are illegal according to definition X, and they said that they report it to NCMEC if it fits their definition of child porn - which is also definition X. Whether definition X includes fictional material or not doesn't matter, the implication is, since LJ think it does include it, that LJ have reported it.