Thing is, if your freze something, that does not stop its internal decay due to radiation, e.g. carbon 14, kalcium etc, its just that the errors which occur arent multipled and the relatively short time is insufficient. But freeze a corpse for a millenia and it will turn into swiss cheese from self radiation, even if it was healthy when you froze it.

Well you are right when you consider that a "mutation" can just be caused by the DNA copy/repair mechanisms. But what if a chemical degradation could also cause it? Perhaps a sample dna lost bases and the sequencing read the resulting base sequence as a new mutated sequence? I am just being skeptical.

The samples were stored at -80°C, where any chemical degradation process would be slowed down immensely if not come to a virtual halt.