Taleb's statement is a much stronger claim than your summary that "Risk-taking appears to be a necessary but not sufficient condition for financial success"
Your summary allows for the possibility that there are other factors besides risk-taking that can help ensure success, and in particular your statement leaves open the possibility that these factors could include working harder, being smarter, etc. etc. Taleb's statement explicitly rejects that possibility, arguing that the factors which distinguish the successful and the non-successful are ones you don't even have control over.
Saying that some smart, hard-working risk-takers failed is not a counterexample, and that's all Taleb was doing, at least in the quoted passage. It's roughly the same as saying that starting cards in Texas holdem don't influence the outcome of the hand because AA sometimes loses. It suggests that he doesn't understand the definition of risk.
You need to know only the basics of Bayesian inference to conclude deduce that if hard work, intelligence, and willingness to accept risk are overrepresented among successful people versus the general population, those traits are probably influential to success.
Maybe being smart and working hard doesn't make you a lock or even likely to succeed. But you'd be naive to think that your chances wouldn't be better than a competitor who was dumb and lazy.
You would also be naive to buy a book from a guy who gives you an explanation for something and conveniently neglects to mention, or perhaps doesn't even realize, that his explanation only accounts for 5% of the variance. A guy who tells you that hard work and risk taking will improve your chances from 1% to 5% in a one field but will improve them from 1% to 50% with less risk in another is giving you information that is actually worth something.
Your summary allows for the possibility that there are other factors besides risk-taking that can help ensure success, and in particular your statement leaves open the possibility that these factors could include working harder, being smarter, etc. etc. Taleb's statement explicitly rejects that possibility, arguing that the factors which distinguish the successful and the non-successful are ones you don't even have control over.