Because in addition to having zero presence online, he was never observed using computers in the school or at the local library, and he showed no interest in doing so at home. I've never come across a case of online grooming in which the victim left no online footprint and was never observed to use computers to access the internet.
Do you remember where you read this? I've always struggled to find much said about his online activity, so any sources where they've elaborated a bit more would be interesting to read.