That is only true if they dismiss the case for lack of standing. In that case, they should also vacate the District court decision. How could a party have standing to defend the law at trial, but not have standing to pursue an appeal? And if there was no one with standing to defend the law at trial, there could be no trial, and thus, no ruling overturning Prop. 8.
However, if they dismiss the case for some other reason (lack of ripeness, etc), then in that case the 9th Circuit ruling would remain in effect.
I covered that in post #34.