Shit happens. I'd be more surprised if he was able to keep the parents alive for 50+ years honestly. That would be beyond belief. He says he started this in the 70's...
My theory:
DJ Short found a hot pheno and turned it into a beast of a seed company, one of the first to do it, he is a pioneer. Then he continually inbred and made poor selections. At some point he lost the parents and had to work with what he had. Which was an inbred line that doesn't carry the desired traits.
If he had the parents he could start over and get himself out of the hole he's in. Logically it doesn't make sense that he still has them. However the rest of the theory seems pretty solid to me, the blueberry is not in the genetics for a reason and that reason is DJ Short, it's the only possible reason. He bred them out.
That's probably going to rub some folks the wrong way because DJ's a legend and shit. IDK to me inbreeding 6 generations and having inferior genetics than you started with isn't worthy of legendary breeder status. He found a super hot cut at the time and marketed it well, credit where credit is due. He did it all in a heavy prohibition and risked a lot, and I respect that for sure.
Inbreeding is generally deleterious, even in flowering plants. Since inbreeding raises the risk that bad copies of a gene will be expressed, inbred progeny suffer from reduced viability. A case study of Leavenworthia suggests that loss of complex traits may be reversed.
www.sciencedaily.com
***Another possible theory could be that he completely lost the parent plants and all his seed stock and tried to recreate it from scratch and there was never any blueberry there to begin with. I'd prefer to go with the first option as it seems less dishonest.