He didn't say that. Cantonese is not a dialect of Mandarin. Mandarin and Cantonese (and many others) are both dialects of Chinese.
Chinese is traditionally only written. A Chinese person would speak in his local dialect, but read and write in Chinese. The local dialect typically had a different vocabulary and grammar from written Chinese. Since these dialects aren't written, only spoken, they're dialects.
Largely due to politics, over the last two centuries written Chinese has gradually shifted such that nowadays its grammar and vocabulary have a one-to-one correspondence with a particular dialect, namely Mandarin. But that wasn't always the case. Indeed, most ancient written Chinese poems don't rhyme in Mandarin: but they rhyme in many older dialects (like Cantonese).
Now stuff like Tibetian: that is a language, separate from Chinese and with its own writing system. This is not the case for Cantonese (and 40 other major dialects in China).
OK, that's a good point, but IMHO the question still stands.
If Cantonese is as different from Mandarin as French is from Italian (though, IIRC, both are Latin languages), then why are they still called dialects?
I see from the rest of the comments that dialect vs. language is political and arbitrary rather than an objective difference.
> If Cantonese is as different from Mandarin as French is from Italian (though, IIRC, both are Latin languages), then why are they still called dialects?
If France and Italy weren't different countries, we might well not call French and Italian languages rather than dialects of the same language. The dialect/language distinction isn't particularly clear cut.
Things have changed for Mandarin nowadays. But traditionally: Chinese dialects are only spoken and are regional, whereas Chinese the language (the written symbols) is national -- and very different from the dialects.
The case can be made that Mandarin is now a language and has subsumed Chinese; but the rest are still dialects.
you may say that French and Italian are "dialects" of Romance Languages. Just like how people wrote commonly in Latin, people in China share a similar writing system but differ when spoken.
Chinese is traditionally only written. A Chinese person would speak in his local dialect, but read and write in Chinese. The local dialect typically had a different vocabulary and grammar from written Chinese. Since these dialects aren't written, only spoken, they're dialects.
Largely due to politics, over the last two centuries written Chinese has gradually shifted such that nowadays its grammar and vocabulary have a one-to-one correspondence with a particular dialect, namely Mandarin. But that wasn't always the case. Indeed, most ancient written Chinese poems don't rhyme in Mandarin: but they rhyme in many older dialects (like Cantonese).
Now stuff like Tibetian: that is a language, separate from Chinese and with its own writing system. This is not the case for Cantonese (and 40 other major dialects in China).