15 degrees is a 27% grade. It's really steep. The highways coming out of the mountains where they have to put bailout lanes for out of control semi trucks are less than 5 degrees. One of the most famous climbs in Tour de France, the climb up Alpe d'Huez, averages 4.6 degrees and maxes out at 7.2, and no one could describe that switchbacked road as the slope of a parking lot.
15 degrees is max slope on a lot of treadmills. I don't doubt he set it at 15 degrees to show off, but not for very long, maybe