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As I understand the science, the fossils may be billions of year old, but the rocks were blasted of Mars much more recently, about 15 million years ago. The answer to the first question is thus covering the second. If they had come from the impact body then Martian gases would not be present.
I could be wrong but they can date meteorites in terms of how long they have been in space using cosmic ray analysis. While in space they are bombarded by cosmic rays which leave a signature that can be measured.
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