This bothered me from the beginning as well. I eventually came to my own personal conclusion that, yes, they have DNA - and - It's completely unusable. IMO if they had usable DNA none of us would be here right now.
Nothing is completely unusable if it still exists, even in bits and pieces.
Some versions of what could have happened.
1) LE with his "Shack" outlook, did not really pay that much attention to DNA, it took a long time to process/divide the mess/mixture of them, and when finally came out to be compared to what is in GEDMATCH, the latter became, essentially, closed to law enforcement. Possible? Yes. They were following wrong clue for 2 years.
2) The BG managed to destroy much of the DNA, so whatever they have is too sketchy to nail him in - but it exists, and it points at him.
3) They do have DNA, and have matches (in CODIS or Gedmatch), but the DNA does not point at their main POI. In facts, it points at a very different person, who does not make any sense as an assassin, and they are trying to sort it all out.
4) It is a very possible situation, btw. A mix of clues. The car points at one person, the witness describes someone recognizable but different, the voice resembles someone but not the person who the witness points at, the DNA implicates someone very, very different. What is found around the south end of the bridge or at the clearing does not belong to any of them.
How the heck you make a case out of it? You probably methodically follow the clues - assign someone to a car, someone to the person pointed at by the witness, someone to trace the voice, and decide how much $$ you can spend on Parabon, or any other company working with DNAs.
But your manpower is limited.
5) the worst situation - the DNA is nonexistent any longer. Or the chain of custody is broken because one of their own, whom they trusted to deliver the DNA, is now among the suspects.
And this is what could have happened, and it could effectively close this case.
But not totally. In this case, the only way is to open everything they have. It will not nail the person, but it will cut any of his careers short.
In the meantime, they could work at the clues that Libby could have had... This part is still very workable.