Toasting has always been a way to stretch the useful life of bread, but it's my understanding that the practice described was more to improve the taste of the beer/wine than as a way to use up the bread. As anyone who has tried to make beer or wine at home can attest, it's not that easy even with modern equipment and materials to make something that tastes good, so imagine how difficult it was on the frontier.
The fact that the words "toast" and "char" both have very old culinary applications suggests to me that the benefits of putting alcohol in contact with toasted or charred wood was well known. That doesn't, of course, mean it was known to everybody. One characteristic of life in remote areas before the advent of modern communications was that if a particular knowledge was not possessed by anyone in the community, it might as well not exist, so certainly not every distiller knew that aging in charred barrels was beneficial.
The other and, probably, more important factor in the earliest days was that in remote frontier communities, barrels and the tools, materials and knowledge to make them were rare and even the most crudely made alcoholic beverages sold readily regardless of their quality. If the producer could readily sell all he could produce regardless of quality, there would be very little incentive to take steps to improve the taste, especially steps that would have been extremely expensive. Aging wasn't so much unknown as it was impractical. It was only when areas became more developed and gained more contact with the wider world that routine aging became practical.
My point is that knowledge or the lack thereof wasn't all that significant, compared to other factors, so the question of how the benefits of aging were discovered is moot.