The powder will not hurt the bore for at least a very long period of time. If the moisture inside the bore achieve a level that the powder became damp, it is possible the bore would be oxidized/rusted somewhat.
How they loaded, ie: wet patch with tallow or dry, we can only speculate.
We know that a wet patch around the ball, with a card wad between it and the powder, will leave the powder unhurt by the lube.
When Taylor did his week's loaded test when using Hoppe's #9 Plus, no extra patch nor wad, there was no damage to the powder, bore nor chamber- although he cycled it indoors and out daily when it was around minus 20 degrees - ie: no rust, anywhere.