I would think the heavy metals bit might be about metals in the coffee itself, from the soil it's grown in. This is common for cacao[0]. What sucks is, iirc, only the EU has limits for cadmium in food imports (or may in the near future?). Beyond that, California requires stuff to have a Proposition 65 label if it's too high in cadmium (but California's limit is significantly higher than what the EU was going to be).

>I'd be surprised if agitating heated water in the presence of particular matter that increased its acidity, then running it through a fine woven filter, didn't have some net effect.

Like sifting for gold? xD

0: https://www.berkeleywellness.com/healthy-eating/food-safety/...