Sure, but this is only a lucrative business because despite the land getting more expensive, the housing units are less expensive—otherwise who in their right mind would pay as much for one unit in a duplex/triplex/etc. as they'd have paid for a single-family home in the same location?
The $/sqft of housing tends to go up as density increases... for the same reason as the article is suggesting: incomes are higher, so people can eat higher prices.
This is only true generally, not within a specific neighborhood, and it's because of correlations between demand and density.
If you look at a neighborhood with mixed SFH and condos, the condo $/sqft is lower than the SFH $/sqft. (To be clear: that's $/sqft of housing space not of land).
Having a diversity of density enables home pricing at different points. Looking only at SFH (as this article does) is missing the forest for the trees, IMO.