The source paper for the article is this one: The half-life of DNA in bone: measuring decay kinetics in 158 dated fossils (available for free). The crux of their methodology is given in the abstract "By analysing mitochondrial DNA (mtDNA) from 158 radiocarbon-dated bones of the extinct New Zealand moa..."
So the article you linked is talking about DNA survival after cell death in fossilised bones. Seeds have not undergone cell death; they're still a small collection of living cells designed to survive in a state of near inactivity until the conditions are suitable for them to germinate. It's therefore not at all clear that we should expect the results from this experiment to generalise to seeds. Doubly so given that, as pointed out by Jonathan, the fossils were effectively held at around 13.1ºC whereas the seeds in the study you mention were found in the permafrost.
Finally, the seeds weren't viable on their own. It was only with significant intervention that scientists were able to get the embryos contained within the seeds to grow.