I think (perhaps?) people are speaking of the major difference between what I'd call "naked metal" and the kind of metal used in common one-gallon jar lids...which is typically a blend of metal (easily bendable) with a heavy enamel (or enamel type) coating.
We've never had ANY reaction with any of our lids (either those on 1/2 gallon canning jars OR those on typical gallon jars). We may have had a slight reaction with the Lobelia tincture (due to the vinegar) when we made it in 1/2 gallon mason jars, but not with the gallon jars. We've been buying/using these from Azure Standard for years:http://www.azurestandard.com/shop/product/2607/