This is obvious selection bias, on several axes. Here are two of them: - There is more/richer information available on people who keep journals, so it's easier to make the case for their greatness. - If someone does great things but no record is kept, they are not acknowledged. I enjoyed the stories of his grandfather, but the flawed reasoning in the argument annoyed me.
I don't think it means much either way as far as whether I'm "great", whatever that means anyway.
I think keeping a journal has been vaguely useful.
That might be a better argument for it.