They must have, right?
Based on my experience it's very much less than half of normal people know how to do this.
Just skimmed the abstract, which says "We find that users switch tabs at least 57.4% of the time." but looking at Table 1, it seems like 64% of people do not open search results in new tabs/windows at all, so your initial guess is about correct.
It's actually quite annoying that middle click doesn't bring up the link.
It seems like a convention that exists for sites that want to force the user into their content even when they want to leave. Unless it is a file download link - I find it incredibly rude.
Source? That's almost entirely untrue in my own experience (my audiences are quite large, quite varied, and not particularly ludite).