Most of the time it will, and perhaps, conditions were just right when he was using it. But true duty cycle is figured at 40 degrees Celsius. And heat dissapation rates are affected by the temperature, and humidity. But the heat must transfer from the IGBT to the aluminum. The heat transfers out of the aluminum faster than it does from the igbt, so theoretically, the igbt and other components could still be hotter than it should and the exact position of the heat detector may stay cooler, since it is away from the other end somewhat. At this point I am postulating why the thermal would not trip. But when it comes to inverters, constantly running a unit against its duty cycle is not recommended because of the cumulative damage/wear done to the electronics. A transformer can be run up to the duty cycle, and other than melting out some solder, little is there that can break...so the effect is not as bad.