He is not right. His original claim was that new authors are mostly wealthy. All he did was weaken his claim until it was (more, but not actually) supportable.
Further, to address your own point, they are not even the representation of successful authors. There's thousands and thousands of successful authors that aren't even near to being close to the gates of the Western canon.
Of course he is right. It's similar story to the early scientists whom all were independently wealthy. And your claim about 'thousands and thousands of successful authors' is just ridiculous when an average person doesn't even read a thousand books in his lifetime let alone considers most of the authors of these books 'successful'.