If you find a wooden chair made from a 10,000 year old tree, that does not mean the chair was made 10,000 years ago. Does that question even make sense? DD
I think that you are asking that if scientists found a flint tool made from a stone that they determined to be a billion years old, how would they know with any reasonable certainty that the tool is of the same age as the stone?
If that is what you are asking, the age of the tool should not be determined using only the "dating method" that scientist used to formulate the age of the stone that was used to make the tool, obviously more evidence is needed to extrapolate the date of the tool.
If I were a scientist, I would not go to Africa to find million year old humanoids buried inches above ground, I would go to Cuba and only after a couple of days or so I am going to find "Desi's" elbow waiting for me, sticking out of the earth. I will write books and become very famous and rich.