Anyone can send you a scary letter claiming anything at any time. Your choices are always A. Ignore it B. Delete your comment or C. Pay a lawyer $500/hour for advice.
People have trouble conceptualizing small probabilities, so that minimum threshold of choosing A and being wrong times "all the money in the world"[1] might be > $500, so it's always rational[2] to delete your comment if you don't want to schedule time with a lawyer.
So in other words, scary letters work because they're scary and make people scared. Sounds like the more people choose C the more these scary letters become self-fulfilling.
Could my comment "Everyone, please block functionalclam.com" also be subject of a DMCA takedown request then?