Is the precision on dating these trees really enough to say that between these two fossils that are over 386 million years old, this one is older by just two or three million years? That seems like a difference you'd expect well within the error bars of however you measure fossil age - is there some way they conclusively can order these fossils without knowing exact ages or anything?
Perhaps they know the stratigraphic relationship between the sites and can estimate that way? (If one fossil is above another and you can rule out faulting or overturning, then you know which is younger.)