Everything in the study makes perfect sense. However one thing that's not discussed is change in culture and expectations. For example the United States is probably the only country where there's a general expectation that grandparents parents and adult children all live in separate homes... and most other countries multi-generation living is the norm. And perhaps some of those norms are starting to float into the United states. On the other hand no doubt the fact that people are not able to live to the traditional American norm is contributing to malaise. Still it feels a little simplistic to characterize everyone's goal is having their own home.
Maybe a way to handle this better would be to look at the size of the home because if a home can easily fit multiple generations it feels like a very different situation. Also it's somewhat arbitrary who happens to be the owner on the deed not to mention key questions like whether or not the home is truly owned outright versus largely in debt to a mortgage
I think you have some valid criticisms, but we have to take baby steps here.
As you mentioned, multi-generation homes are not the norm in the US. If there is a change in that norm, it's worth investigating why that's happening, before we conclude that we need to adjust metrics around it.
Purely anecdotal, but every multi-generation home in my (American) family is the result of disability or student loan debt - multiple generations live together, but that doesn't mean they want to.
Also remember that this new metric is meant to be more informative on wealth, generally. Those other generations living in the home don't typically share in the wealth the home confers, not until someone dies at least.
There is not much difference between the US and Europe in that. In the Nordic countries, children move out earlier, while multi-generational households are more common in Southern Europe.
Right now adult Americans can freely chose to live with their parents, and overwhelmingly chose not to.
The article links ownership to costs. Attributing diminishing ownership as "norm float" needs some justification. I suspect multi-generational norms seen elsewhere are rooted in poverty and a lack of opportunity.
Suppose I told you I am losing weight because I cannot afford food, I don't think you should speculate I chose to go on a diet.