I don’t think that’s as big a problem as the media makes it out to be.
Sure, there will be trace DNA left behind, especially in common areas of the house, likely less in the bedrooms. But if there’s another person’s DNA mixed into a victim’s blood, that’s significant. If that same DNA is mixed into the blood of multiple victims in multiple locations, that’s a giant flag. And, if there are drops of blood of the same DNA separately, that’s a smoking gun.
I suspect that they have strong leads they’re not discussing. The fact that the FBI just put more agents on it does not say to me that they’re running out of leads…it says that they have information they’re actively pursuing.
Also my own assumption, but I think If it was a random psycho thrill-killer, there would have been a repeat by now.