That isn't how I interpreted it. While they may have conclusive evidence that Caylee's deceased body was in the truck of the car what evidence do they have that Casey killed her? If the body is indeed the missing piece of evidence then I'd still have questions about how they would prove Casey killed her or covered up her death. It is unlikely that a body would prove who killed her, or at this late date even how she died.
I have a lot of faith in LE in this case. However, it isn't unusual for murder cases to go "unsolved." The FBI stats show that only about 60% of murder and nonnegligent manslaugter cases are "cleared" on average in the US. That leaves a lot of people getting away with murder.
http://www.fbi.gov/ucr/cius2007/offenses/clearances/index.html#figure