In this particular case they had the suspected killer's DNA
They ran search algorithms through a database where people allowed their DNA to be searched via the terms of service. They found family matches. They searched through the ancestry of those two matches, found a common ancestor, and then searched down from that ancestor, finding descendents of about the correct age. They aren't going to have DNA for everyone to search up, and back down so are going to have to dig through marriage and birth records, the kinds of stuff that genealogy experts do all the time. In this case they determined that it had to be someone from one family, and that family only had one son. It turned out the son lived close to where one of the bodies was dumped. They trailed the guy for awhile and picked up a soda cup that blew out his car window. They tested DNA from the cup and it matched the DNA found in the rape kit and on a pair of the victim's jeans that were found in the victim's van.
This case is interesting in that they don't really have any direct physical evidence for murder. It would seem rape is almost a slam dunk and maybe you get to murder for the wife. The husband's body was found 90 miles from the wife's body with no evidence to connect the guy. From what I heard I would guess they convict on rape, probably the wife's murder, but not the husband's. And I wouldn't be totally surprised if they didn't convict on her murder but I don't know the details of what evidence they do and don't have.
So in this case they had semen, and used these genealogy records to identify the owner of the semen. The same thing happened in the Golden State Killer case.