Even if the perpetrators are telling the truth, there is a world of difference between 1999 forensics and 2021 forensics. It might also be that a hit-and-run wasn't initially suspected in Erica's case, so investigators may not may been looking for evidence. If they were, they might not have not the exact location where she was hit, and that would have made finding evidence very, very difficult.
In Summer's case, if she had died in a hit-and-run, in all likelihood it would have occurred very close to her family's property. That area was scoured by investigators. There's no doubt in my mind that 2021 forensic science would have found some evidence of a hit-and-run if that had occurred.
A scenario in which someone hit Summer, thought, "I've got to get her to a hospital," took off with her, realized she was dead, panicked, and disposed of the body would not be out of the question, but there would have been evidence: skid marks, blood, a large quantity of Summer's DNA where she hit the ground, etc.
Some things can be ruled out or ruled utterly improbable by a lack of evidence. The two scenarios that would not leave much evidence would be an abduction from the yard or a wandering away, and the latter would not make sense with the timeline provided by Candus. If you believe that Candus is telling the truth or only fibbing slightly about the timeline, then you're down to an abduction. If Candus's 5–10 minutes is really 30–45 minutes, then Summer's wandering away becomes a viable possibility.