Well, in fact it’s not completely true. You can either digitize your map in terms of highlighting the walls and entering their materials (not always known, btw), which NetSpot doesn’t support for now, or alternatively you just follow our survey tips and take more samples and more frequently: /help/tips-and-hints-running-a-wireless-site-survey/
The more samples there are in the map, the more true-to-life the resulting heatmap will be. Even if you don’t know what a specific wall is made of, just measuring your WiFi coverage on both its sides will provide you with actual signal data, which is used to build your heatmap in NetSpot.