The article is generalizing a bit, and focusing on just one aspect of boomer housing matters.

For example, it's also quite common for baby boomers to hold onto their existing 3-bedroom homes, and in the process restrict housing supply of what should be affordable, reasonably-sized homes for millennials seeking to put down roots and start a family. I don't think there's anything wrong with this BTW, as it can be quite prudent to do so.

I think people were gambling on baby-boomers moving back to the cities and "downsizing" to a decent 2 or 3 bedroom condominiums. Instead, those units ended up selling to cosmopolitan millennials and Gen-Xers.