It wasn't mentioned, but I guess the money is being reinvested in equities. 5% is a stretch goal seeing as the savings rate in the US is around 1% (or less). Inflation there is running at about 2-3%. I see no talk of capital gains taxes or broker fees in there, so I'm a bit skeptical when people throw around instant, magical compounding calculations that net a million dollars.
Oh, it's 5% after inflation which means the money is compounding at approx. 8% or so. That's a very lofty goal!