I would say assuming people from Guangdong province, and majority of them don't speak Mandarin is an exaggeration to great extent. Of course they do, it is imperfect with heavy accent. But I won't go that far claiming they don't speak Mandarin, as if saying all English speakers in India don't count, that is just hilarious.
It is definitely your claim about the 300 million Chinese that they don't speak Mandarin, that is why the whole conversation happens. Otherwise, it won't add up, even assume majority of Guangdong province don't speak Mandarin at all.
