How do they know it was a 70 million year old dinosaur fossil? Well, it was in a 70 million year old rock. Well, how do they know it was a 70 million year old rock? Well, because it had a 70 million year old dinosaur fossil in it. See how this goes?
Not to mention that many many dinosaur bones contain soft tissue....
....which should be “fossil” if truly millions of years old.