There is something that is observable from the article. The fact that there are particles about .01mm in diameter that are magnetic and found in rain gutters in Europe. That is observable evidence. However, you are correct. How it got there was not observed. I wonder if the researchers even thought of the FQs you asked in your post.
Great article! Now I want to go and dig through the sediment in the rain gutter I put up on my back patio. Now where’s my magnet?