There is nothing certain about it at all. Yesterday, on another thread, I told how my own skin cells could easily end up - innocently- in a kid's underpants. I'll go through it again briefly here.
I work in an elementary school. On a few occasions I've helped kids get dressed for recess, helping them on with their snow pants. In this process I may have to pull up their regular pants to get the snow pants on. That would mean I've touched the waistband of their pants. I've also helped kids wipe spilled milk from their shirt or blouse in the lunch room. In both cases, I have to touch the kids and my skin cells transfer. The kids could then transfer the cells further by touching where I touched and then putting their hands in their underpants. Now, suppose one of these kids is killed in their home. We now have my dna in 3 places on 2 articles of clothing, including underwear. Am I almost certainly the killer?
Do we know what kind of cells we are dealing with in the panties? It's being called "liquid" dna. Does that mean it came from blood, saliva, semen, or does it mean it was transfered in a liquid?
The more important point about the dna is that there was enough to do normal analysis, rather than lcn analysis. But we still don't know how it got there. If it turns out to have an innocent explanation, we are right back where we started. If it turns out to be the dna of someone who was a child at the time, we are back where we started.
Is this enough for a good defense attorney to create reasonable doubt in the minds of jurors? Sure. Is it enough, as a matter of logic, for detectives (or the DA) to cross someone off the suspect list? No.