Reading their responses-the gist is that unless the fictional drawings are unmistakably minors depicted in sexual situations it isn't worth making it a violation and as such CP.
Especially when we get into odd territories such as art styles, intent, etc. (CSA survivors for instance make closure with their abuse by talking/drawing about it and moving on.)
On top of that you have a decent amount of people who are against this measure on the basis that the time, money, and energy spent prosecuting people making cartoon porn is significantly better spent going after individuals and organizations who prey upon, kidnap, abuse, rape, and distribute said activities of REAL kids online.