Are you asking why this is not a mandatory-reporting-of-child-abuse situation? The purpose of mandatory reporting laws is not to bring justice against the abuser, it's to protect children. So what they want to know is whether there is currently a child or children who are being abused. If you call them and say "this guy told me he had a sexual relationship with a 17 year old 10 years ago," what is there for them to investigate? That kid is now 27 years old and thus is no longer a child in danger of being abused. I've never made a report so maybe someone more knowledgeable can correct me if I'm wrong, but they have to have a specific situation to investigate. They're not going to patrol the guy's neighborhood, workplace, church, etc. and investigate whether every child they find has been abused.
Also, age of consent laws vary by state, so we don't know that this relationship was even illegal. And another point where someone more knowledgeable can correct me: I'm not sure whether having a sexual relationship with an older adolescent, while illegal if the adolescent is under the age of consent, is considered to constitute "child abuse" for the purpose of mandatory reporting laws.
That said, this seems like a poorly written case, because if they think there's insufficient reason to go down that road they should have made it more clear.