I would vote to include the word "primarily" and let the Hugo administrators sort them out. :) Thus, the review column or a science article in Analog wouldn't move it out of the fiction category; a short-story in Locus wouldn't move it out of non-fiction.

I understand that judgment calls can suck, but there are some days that you have to assume that people aren't complete idiots, whatever evidence might exist to the contrary. :)