Turns out I was wrong. I did further research, and Numeris does require stations to be subscribers to show in the release data, but they have a special policy for "spill" stations... in other words, out of the central market stations that make the book. To show in a market they are not located in, they must be a subscriber in that market.
So it appears that KWPZ is subscribed. And if they are, it's either because they have found a way to attract Canadian advertisers, or they believe that US advertisers will be interested in knowing that the station has an audience in Canada that may patronize across-the-border businesses.