If DNA has a half life of about 500 years, how can old seeds be planted?
That half-life was for temperatures of 13.1ºC. In more ideal conditions, such as for temperature of −5 ºC or drier conditions, the half-life would be longer. The Wikpedia article in fact states that the seeds that were 31,000 years old were frozen under permafrost.

The original article estimates that the half-life at −5 ºC (for a 30 bp fragment of mtDNA from bone) would be 158,000 years. I'm not sure how well this data can be extrapolated to seeds.