I see what you are saying. And I agree that in most cases, you can't rely on DNA alone. Although child cases a bit different just IMO, since consensual sex can't be argued.
But in this case, I don't believe they just have DNA. They have plenty of circumstantial evidence that puts him in her path during this time. Her body was found almost immediately after she went missing. You have a suspect who admits to seeing her in the time directly before she went missing/was found, you have his DNA, you have his statements that can be used against him. I'm sure they have more than this, since they've been tight lipped. If he has any of her DNA on his clothing from that night, I think he's 100% done without any doubt. Or, if their DNA is mixed. I wonder if they fast tracked the DNA from her autopsy and rape kit to ID a suspect, and now the rest of the evidence is being tested. I'm curious as to how all that will turn out.