My thinking that Iain was the main target was more due to the quantity of his collection and length of collection in comparison to Fillip.
I may be incorrect, but I believe if the content included depictions of real humans, than there would have been mandatory prison time instead of just supervisory restrictions. I've worked on the technical side of similar cases within the US, but UK law has added complexities with these additional restrictions on content, and their overall British asshattery. Even on the US side, there has been some weirdness on the prosecution side with terms/legal definitions/ etc, so Fillip's explanation does seem to lend some credibility that maybe he and Iain were only collection drawn images.
They could also just both be dirty ass pedos that got off easily, maybe for a first offense, or snitching a source.