Black powder burns hotter than smokeless and the barrels on a gun shooting BP loads are almost untouchable during a normal skeet round. Shooting BP rounds at a faster rate than that in a SxS with soft soldered ribs with the likelihood of them melting and we wouldn't have many of the old guns around in original shape anymore. If the solder were to be at or near the melting point, so would the plastic shell casings, paper caseing would at least be charred, plastic sight beads would deform or melt and wooden forends would flame. It's not much of a trick to get the excess oil in the forend of a fast fireing firearm to smoke under the right conditions, but bringing a set of shotgun tubes up to 350-400 degrees by fireing is. Soft solder isn't perfect and not all solder joints are either. They do have voids, faults and barrels do flex somewhat on fireing. A firearm is a machine and they do breakdown and need repair. Ribs do get loose..
As for high strength high heat brazing solving the problem..I recently fixed another loose rib on a single barrel gun that was originally attached at the factory with 'high strength high heat brazing'. Both repairs were done with soft solder and are still running fine.