It seems to me, if true that DNA has a roughly 500 year half life, then the discovery of Dino DNA would absolutely disprove that dinos have been extinct for millions of years. The absence of such discovery doesn’t conclusively tell you anything.
The article referred to this a geographically constrained set of samples. So the 500 years pertains to those samples. On the other hand since temperature is an influence, I’ll bet that the DNA of some of those frozen mammoth carcasses is probably still in pretty good shape.