They say to avoid eating larger older fish because they have higher concentrations of mercury, but that mercury from fillings does not accumulate in humans over time. We aren't that different than fish.
They say to limit fish consumption because the mercury damages the body/brain/fetus, then they say mercury from fillings passes right through the digestive system without being absorbed. Bacteria inside the human digestive system, from piehole to ahole, converts mercury to M/Ethylmercury.