That could well be brazing, but note that it is on the barrel, not the receiver! As a generalization - i.e. not a lwa written in letters of gold on a mighty slab of granite set atop a high mountain - just a generalization - barrels are effectively a high-quality mild steel and are not hardened by heat treatment. But receivers often use materials such as nickel-steel alloys that are quite touchy about heat-treatment, and any tempering would be lost at brazing temperatures.
So, as a generalization, one may say: barrels can be brazed, but receivers should not be brazed - not even hard-soldered.