they had DNA evidence... That should be enough ...
I have my nephew's DNA (blood) on my living room carpet from when he once picked his nose too deeply (he's 3).
If (God forbid) he goes missing, am I guilty of kidnapping and murdering him?
Yes, there was some DNA evidence. The question that was never answered is, when was it deposited? Absent any evidence that DW was ever in the VD home and the assumption that whoever murdered Danielle took her from the home, one cannot reasonably conclude that the evidence was deposited at any specific time.
The evidence simply does not support the verdict. What the prosecution needed to do was identify the other DNA evidence in the VD home to check for other suspects.