Need some help, melted fuel line!

I have not had any problems with the NAPA/Dorman (it's all the same) fuel line repair components. I have used the single-pack GM repair line, but usually I use the full kit with utilizes a "gun" to install the barbs and is double secured with bands. I have never tried heating the lines, since I use the gun. I wonder if that process is altering the material enough to become attached by solvents in the fuel? This is actually the first time I've heard of heating the lines install the barbs. Are you sure this is an approved procedure? Or is this a shop workaround because somebody lost the "gun"? Or maybe they couldn't figure out how to use it? It's quite the puzzle until you figure it out the first time.

This is what I always use: