If you have too low a pressure your flame will melt your nozzle.
The non-combustible gasses, ie Nitrogen, mixed with the gas, lifts the flame up above the nozzle so it wont melt.
Alumina has a melting point around 3000 degrees F, and the flame can reach 5000 degrees F, or over 20,000 degrees with the Hydrogen Fracturing Process.
Alumina was chosen because of the high melting temperature and that the holes will maintain their size and shape, whereas brass, copper, and so on will erode and widen the hole after some use.
The quenching tube is for safe transportation of the gas, not to act as a nozzle.
Stan says all of this clearly, if you want to understand him, you should listen to him