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'.
That's such a silly standard of success to think that one must be entered into the Western cannon to be considered successful. The number of authors that have made good money writing books far, far exceeds, by many orders of magnitude, the number of authors in the Western cannon.
In the 20th century, there have been hundred of thousands to a million different titles published. If even 10% of those authors made enough money to be considered a living wage, it absolutely dwarfs the number of authors in the cannon, which is on the scale of 100s, not thousands.
If the cannon is your standard, your standard excludes almost the entire industry.
If your standard of success is 'making good money' instead of leaving lasting impression and still being read hundreds of years after the author's death then you are still wrong. Only a tiny amount of all authors were able to make _any_ money from their books, let alone enough to live comfortable lives. Book publishing is like professional sport or entertainment, only the very top make any money. Hence if you are already wealthy you can keep trying and maybe finally you make it big. If you are poor you don't have that luxury.
