The article mentions that he is an importer as well as a collector and seller. The reference to omitting marking to increase value sounds like they are talking about import marks, not serial numbers. Not sure which law, but I would guess the GCA of 1968 requires importers to mark their identity on guns they import. People who collect mil-surp guns are often willing to pay more for a gun that doesn't have an importer's name stamped into it.
I guess we just stay tuned.