They would have almost certainly taken samples from all the victims and all the roommates (DNA and fingerprints), so they could compare those to any samples they recovered. That's when they would have come up with unidentified DNA, which is the only thing relevant here (which is why control samples wouldn't have been mentioned in any filings).
The DNA on the sheath would be problematic if he didn't drive a similar car, his phone activity was normal, he didn't resemble the person the roommate saw, he hadn't driven in the vicinity of the area on multiple occasions, he didn't act like a fricken weirdo with the gloves and trash at his parents house, and didn't fit the profile of an organized killer in every single way.
There's going to be much more evidence than we know thus far though, I'd bet the farm on it.