The passage tells us that rock varnish forms on a rock that has been cut. We have evidence that there is organic material trapped under the rock varnish on a rock in a given monument, and that since this organic material is old, the monument that the rock is part of must also be old.
I can think of two possible flaws here. First, it seems possible that some organic material was already on the stone before it was cut, and the rock varnish formed over that material. If this were true, the age of the material would have no bearing on the age of the monument. Second, the time the stone was cut is not necessary when the monument was built.