I'm going to talk to someone at BF about this at my next opportunity. It certainly is close enough to the original to make the claim credible, would be my guess. In other words, it's as close as they can make it.
In 1874, there really was no such thing as GNS. It wasn't possible to make it, so if it was grain spirit it was technically whiskey, even if it wasn't exactly "straight bourbon" as we would know it today.