Then by what method are they dating it?
They didn't say, but the usual practice would be to use the usual geologic dating and apply that to things found in that stratum. If someone had actually buried the hatchet there, the hatchet might be of more recent origin, but if it had been lost in the mud it would act like an ordinary rock.
One or more of these, most likely: http://www.origins.tv/darwin/datingmethods.htm. Note that there are many, many types of dating methods, which rely on entirely different methods and procedures and underlying physical processes. Be sure to ask the next young-earth creationist you see how they all manage to be "wrong" or "unreliable", yet produce consistent "incorrect" results with each other...