Looking at the included chart (below), I don't think the growth in "unknown" is the primary factor of Window's decline in market share. The chart does show, over the last two data points, that Mac OS and Linux have increased at the same time that Windows has decreased.
This is all but one data point anyway, so it's not fair to draw a long-term conclusion based on that regardless.
Yes, that's pretty much what I said in the line of text under the graph.