I agree that if they prove a tight correlation it means something. I can't see anything about that in the links though, so is this an article which proceeds from the assumption that the correlation with earth magnetism is very tight(if we assume correlation is tight then we can conclude magnetic field has decreased 30%), or does it prove the tight correlation as well. There is statistics involved and I can't see what kind.
Testing that correlation can also be done with experiments today with new pottery. Have such experiments been done? I take it that fast variability of the earth magnetic field exists is already a given, because I doubt if old pottery by itself will carry enough weight to prove that the magnetic field can change quickly.