I don't think that's the case. The article specifically mentions multi-family units:

> Austin, TX presents the clearest example of this trend. From 2021 to 2023, an average of 9.9 multifamily units were permitted for every 1,000 existing residents in the Austin metro, by far the highest rate among the nation’s 50 largest metros. As these new units have gradually reached completion, the median rent in the Austin metro has fallen by 7.3 percent over the past year, which also ranks first among the top 50 metros.

That depends. Builders build what there is profitable demand for. If there is a lot of demand that means the high priced high end homes are in demand and builders won't build anything else. (unless forced) However there is only finite demand for such things so and as you build eventually someone says "can you make it cheaper" and some builder - having no other customers is forced to build something lower end just to get any work.

Now builders won't accept a loss (in general), so that does limit how low end they can go. However the other half of this is that most people moving to something high end just freed up something lower end for someone who can't afford luxury. (most people cannot afford a second home and even those who could afford it want to spend their money on something else. There are also a small number of junk houses that are so worthless they end up destroyed, but this isn't common)