SSG, flex/assumed takeoff is not the same as "hot & high".
Hot & high is high density altitude takeoff. Low pressure at higher altitudes in involved there as well, leading to higher required airspeeds for the same groundspeed. Not only you have lower thrust (if it's hot that high anyway), you need to accelerate to higher groundspeeds as well for the same true airspeed you need on takeoff. That's not the case for reduced thrust takeoff at low altitudes.