Someone was already reported that but that time it did not even nullfied the fire type. It is nulfying the fire type but it does not boost fire type by %50. Tested it. 252 SpA Charcoal Chandelure Fire Blast vs. 252 HP / 0 SpD Machamp in Sun: 402-474 (104.6 - 123.4%) -- guaranteed OHKO. I mean sun and flash fire doing the same thing in theory. I arranged all the ivs for myself and maxed the machamp's stats.

it increases the attack power (which gets calculated in the amount of damage you do)

It does not increase the damage by 1.5

 

if(this->GetFlashFire()){

atkPower = atkPower * 1.5;

}