I'm sure that it doesn't mean ALL Chinese nationals thinks HK show business is rubbish. It refers to the some 30 Chinese artists whom had signed for the petition.I, for one, am hugely offended by the "which proves their views about Hong Kong show business" comment.
Do you have any idea how popular HK-produced entertainment is in mainland China? To hint that people from mainland (whether ordinary audience or artists) think Hong Kong show business is rubbish is just too hideous.