It was a simple statement the defense made in court early on after they had received that evidence in discovery. I'm not sure why they challenged it, unless they just wanted to make the prosecution state again that they had no DNA evidence for GW4. His defense team has made some motions that indicate they want to make sure there's no new surprise evidence introduced during the trial.
It could be that GW4 just got lucky and didn't leave any DNA evidence. It doesn't mean there isn't other evidence pointing to his involvement in the murders. Prosecution wouldn't have brought charges against him if they didn't think they could make their case.
Agree, it's interesting to note none of the other defendants have tried to make a similar argument.
JMO