The commentary is only that,commentary. The code rule is quite clear however. Receptacle required within three feet of the outside edge of the basin. Example: one vanity counter + two sinks = one receptacle, provided that the receptacle is not over three feet from either sink. I would install two myself but this may not have been possible here or the homeowner did not want another receptacle in the mirror. That commentary from the handbook should be changed or removed as it seems to cause more confusion rather than being helpful.
