I would have to agree that in the transition period from BP to Cordite, barrel makers overcame the weaknesses of the older steel by beefing up actions and barrels. More rigid and elastic steel alloys that allowed thinner barrels yet again were not common yet during the transition period.

Which still means older BP guns should not be shot with nitro powders, even if peak pressure is within specs. It is of course possible to overcome weaknesses in barrel steel to a degree by just using more of it - but the old BP barrels used relatively in-elastic steel AND were thin. Some BP barrels (especially on BP shotguns) were paper-thin damascus barrels. Very pretty and extremely light-weight but not very rigid at all.