This is the definition Ive found people use when they bring up cultural relativism. Whether informally with stuff like “you cant judge them, its just their culture” or when more fleshed out. As far as I can tell, no, ethicists for and against cultural relativism are discussing something quite in touch with reality.
The author is an important figure in metaethics, its much more likely theres something you missed than him being wrong about a basic definition in a field he is an expert in.