I believe here they’re referring to the half-life of free DNA. In cells (plant or human) the DNA will be bound by a range of different proteins, wrapped up and

I believe here they’re referring to the half-life of free DNA. In cells (plant or human) the DNA will be bound by a range of different proteins, wrapped up and packaged (e.g histones, but there are many other types too). Presumably the half-life of these proteins is shorter than free DNA but their presence will still change the overall half-life. Environmental conditions are another factor. The inside of a seed could have a more controlled pH, temperature, humidity when compared to the surrounding environment. That would also change the effective half-life.