He advises investing in the SP500 because for people who don't know what they are doing he doesn't see a better alternative. And he's right. And sure, it will

He advises investing in the SP500 because for people who don't know what they are doing he doesn't see a better alternative. And he's right. And sure, it will almost certainly track the economy in some sense over decades, but it need not do that over 10-15 (or 17 in his example), and it isn't some law of physics type situation. The underlying factors that drive returns on capital through time (strong property rights, reasonable labor laws, stable government) also likely drive economic growth.