In my experience in the recent market (in a HCOL city in the U.S.), rentals (even only compared to the costs in homeownership, i.e. ignoring principal payments) are much cheaper than mortgages for an equivalently sized house/unit. That's the assertion of the article, and it rings true.

You're right that the bulk of the mortgage payments, especially towards the beginning, do not go towards building any capital, which makes the investment even less appealing.