Which would be solid advice, but he put there is no limit. That's the part I'm questioning.

You'll never get anywhere without self-improvement, of course, but there's no sense pretending we can all be a prodigy like von Neumann - whose natural intelligence was shocking even to the top professors of his day - if we just work hard enough.

There very much is a limit, but one need not be deluded to be motivated or successful.

edit Perhaps the point is meant to be read as 'you can always improve yourself further', rather than 'there is no ceiling on your abilities'. Which seems far more reasonable.