Their point is that an increased housing supply should shift it to a buyer's market - it's not just how many tenants there are but how many housing units vs how many tennants.

Whether it's a buyer or seller's market depends on supply!

If - hypothetically - I had a ton of money and buying another house or two or fifteen wasn't a big deal, wouldn't there be a clear-ish signal that I should stop my demand for more housing lest too much supply screw with my income? I would also have an incentive to deploy some of my resources/capital to making sure that the supply of housing is juuuuuuust right for my extractive needs.