Your comment is completely misinformed.
From the National Institute of Mental Health:
Mental disorders are common in the United States and internationally. An estimated 26.2 percent of Americans ages 18 and older — about one in four adults — suffer from a diagnosable mental disorder in a given year. When applied to the 2004 U.S. Census residential population estimate for ages 18 and older, this figure translates to 57.7 million people.
I don't have the figures for 2004, but I do have the figures for 2012 regarding homelessness. From the US Department of Housing and Urban Development:
On a single night in 2012 there were 633,782 homeless people in the United States[...]
Dividing by the US population in 2012 (312.8 million), we get 0.00202615728, or, 0.2%
So 26.2% of Americans are mentally ill, and 0.2% of Americans are homeless. So no, it's not a "positively idiotic statement." The mentally all are all around us, and perhaps the reason the study can't pin down why they're dying younger is because people are under the impression that you can easily spot someone who's mentally ill. Yeah, a lot of homeless people are mentally ill. But about a quarter of everyone is mentally ill, and trying to put the mentally ill into a box means that most of those people will go untreated because they'll be ashamed of their disease.