I mean, imagine what housing in Toronto would cost if they didn't build those 100K+ condos? Pricing is composed of supply and demand, and the article is saying that holding demand constant, increasing supply lowers prices. Adding a marginal unit to the housing stock kinda has to result in lower or equal prices in the static case.

Toronto is growing insanely quickly, and supply can't meet demand. New cities are great, but a large number of people want to move to big and established population centres.