His central point is that the "rent" you pay via the costs of homeownership are often approximately equal to the costs of actually paying rent. Since principal payments are on top of those costs, that is money you _can_ do something else with if you were renting, in this analysis.

Of course, that part of the analysis ignores the potential capital gain of the property value, which is often one of the appealing financial factors of homeownership. But the article also addresses that.