It doesn't say how they got this number in the piece (unless I missed it), but it's likely more than 5% if they are, say, counting the OS by user agent strings hitting a particular tracker. Linux distros use different browsers and they don't report the OS in an accurate way all the time.
For a long time my UAS just said "Firefox, the version #, NT-based" or something like that, but now it reports Linux properly... I haven't been paranoid enough to use a agent switcher lately.