The article incorrectly says Immunoglobulin-G is unique to the novel coronavirus. That is clearly wrong. A particular TYPE of Immunoglobulin G can be unique to the novel coronavirus. It's like saying Budweiser makes a unique product called beer. It does not. Many brewers make beer. Budweiser beer is unique based on certain variations compared to other beers. The same goes for the Immunoglobulin G used in the antibody test.
That's not what the article says. Unique doesn't even appear anywhere in the article. Going deeper into various classes of Immunoglobulin-G isn't necessary, given the context anyway. It speaks about both immunoglobulin M and immunoglobulin G. The article says "Immunoglobulin G is specific to the novel coronavirus." The context here is basically just making a general comparison between H and G, during a certain stage of the infection process. The context here doesn't require digging into the details, of various classes of Immunoglobulin G that exist.