I'm not a geneticist, so I can't explain the fine points, but it would be similar to the way they recently identified the "Buckskin Girl" whose body was found back in the early 80's. LE can ask one of these services that maintain DNA databases from people who submit it. They're able to sort out which people whose DNA they have on file are a similar or "familial" match to the UID or killer. They then contact that person or persons and ask them to help identify who the killer might be. They'd have to go through a process of elimination, checking backgrounds, etc.
My other theory is that the recent tip (I recall the sheriff said they received it 6 days ago) came from another retired LEO who perhaps worked with him in the past and maybe always thought him hinky. Could have also been a former co-worker from another job.
Just guessing, but he may not have had a lot of close friends, so outside of his family, co-workers would be next in line as people who knew him.