Of course temperature changes but if it stayed below freezing the whole time then it’s preserved. Obviously that would depend on quite a bit of luck. This also explains why it is the only sample ever found.
Food deteriorates in the freezer. So you are positing that the temperature for this particular patch of earth remained constantly at an extremely deep freeze for 70 million yrs?
Btw, how do you know that even if it did remain in a constant deep freeze for 70 million yrs, it wd be this well preserved? Has there been a test case?