Interesting, given all of the articles about increasing consumer debt. I'd guess that a lot more people are motivated to do this, with mortgage rates fluctuating upward so quickly over the last couple of years.
Anecdotally I know that none of my neighbors are mortgage free. However I also have a few friends that had major cash windfalls things like startup exits or time working in hedge funds, each of them owns 3-5 houses outright as investments.
So, with this small sample I can easily see that of the total number of homes owned by friends/neighbors, 40% being mortgage free sounds about right, but far less of my friends/neighbors are mortgage free.