"Being a willing participant" makes it a much lesser form of rape. And I believe the sheriff would've qualified his statement if there was any evidence whatseoever that this innocent little girl participated in her own abuse by the man who raised her as a father, in any way, which would make it a statutory rape type of charge.
Being a willing participant - your words - is different from being a passive and fearful victim, who does what she's told or fails to scream out, etc.
I am offended by the suggestion that this is a possibility in this case. This baby ran away for a reason. She didn't run with away with her abuser. While a vulnerable child who is manipulated by a predatory older adult into a "relationship" is absolutely a victim, there is a difference. It's still horrible and criminal. But a little girl raped by the man who raised her is different from a vulnerable child convinced she's in love with her molester.
For that reason, rape is punishable in TN by a possible 8-30 years and aggravated statutory rape of the kind you may be thinking about would be punishable by 2-12 years.