I think the kidnap victim being 18 was known since the beginning, the issue is that they were not 18 when Miller allegedly began their relationship. It doesn't stop being grooming when the victim turns 18.
The accusations can have merit while also being from people who are themselves sketchy. If Miller is guilty, it just means they know how to pick their targets, people with bad situations and lack of support who are vulnerable. If Miller really has a cult like persona, that's generally how cult leaders operate.