You're just going by intuition, which can be misleading.
Guangdong province alone is 100 million people. Some of whom are Mandarin speakers (1st or 2nd or 2nd+ language). But many are not, not even as 2nd or 2nd+ language speakers. And their predominant language is Cantonese. Many people grew up before mandatory Mandarin education was instituted. There's a similar situation in other huge provinces all around China, especially southern China.
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.
Guangdong province alone is 100 million people. Some of whom are Mandarin speakers (1st or 2nd or 2nd+ language). But many are not, not even as 2nd or 2nd+ language speakers. And their predominant language is Cantonese. Many people grew up before mandatory Mandarin education was instituted. There's a similar situation in other huge provinces all around China, especially southern China.