I think you're misusing the term "herd immunity". Herd immunity implies that the risk of an non-immunized individual contracting a disease is reduced because his chances of bumping into a sick individual is low.
I don't think this applies here, insofar as we're not talking about an infectious disease.
I think it's a good analog if he's suggesting that all the cows with magnets will eat up all the metal leaving little for the ones that do not have magnets. No idea if that's how it works though.
I don't think this applies here, insofar as we're not talking about an infectious disease.