You could do this by making a polygon covering your whole network area (in this case, the whole city of Ottawa) and then using the Erase tool to subtract the 15 min walking service area (SA) polygon. To do this, you would have to merge all of the individual 15min walking SA polygons into a single shapefile, as Erase can only use one polygon for the "Erase Features" input. Once you have your beyond-15min walking SA polygon, you can use this video's Spatial Join technique to calculate the total population of this under-served area. Hope this helps (and works!)
