I agree, but it's interesting that it was different in the Luke Mitchell case - three years after he was convicted, DNA from another man was run through the database in relation to a completely separate event and threw up a match to a condom found near the body. The Crown released that information to the defence. Even though the police refused to investigate further on the basis that the case was closed, the match itself was disclosed.
That was, however, more than 10 years ago and we know the situation has just got worse since.
I suspect, like the Kevin Nunn case, they already know (or strongly suspect) what the DNA would say and are simply sitting on the information - after all, in their eyes, two convictions are better than one. It's not as if Michael Stone would get a huge compensation payout, given the new approach to that, so the only reason for keeping him imprisoned is that the conviction, itself, is the "prize" for the system and they don't want to give it up.