There's a difference between having the humility to admit that you might not be able to hit another home run, and between claiming that "all the good ideas are taken." At best, the latter is an admission of a lack of a desire to even try anymore, at worst it shows a stunning lack of curiosity and creativity.

To assert that "all the good ideas are taken" is ridiculous on its face. How can anyone possibly know that? How can anyone even know that the number of possible good ideas is finite?

It's a thing that has been said by various people throughout history and it has always been a ludicrous assertion before. Why does he think it's different now?

If what he meant was "I'm out of good ideas", that's different, of course.