It should have been phrased, "your return on investments can be the sole source of your retirement wealth."

I think what they mean is that the bulk of your wealth will be the capital that you put into your investments with a reasonable amount of interest/capital gain.

In context, it's clearer.

Don't be banking on that elusive big investment win to save the day if you're not otherwise on the trajectory you want to be on. Of course, you hope your investments will preserve your savings and augment them. But outsized investment gains won't in general get you there by themselves.

(Also remember that the 10 years since this was written have been something of an outlier for the stock market.)

imo it means that you should also own a place to live in.