This blog post is highly misleading. The only reason you made $6k in a month with index funds is A) because you would have already had about $60k in the account and B) because indexes were recovering from a large drop just before that one-month time horizon. To give more perspective, the 3-month return for your most heavily weighted fund is about half of the current 1-month average. To make $6k/month assuming average annual growth (not something you can always count on, but let's keep it simple), you'd need closer to $900k already in your account.
Should people invest? Yes. Should they invest in index funds instead of day-trading? Almost certainly. Should they expect $6k/month returns from whatever change they scraped from underneath the sofa cushions? No.
Should people invest? Yes. Should they invest in index funds instead of day-trading? Almost certainly. Should they expect $6k/month returns from whatever change they scraped from underneath the sofa cushions? No.