This is an interesting layer of interpretation, but I'm not sure I buy it as revealing actual intent of the park designers, for a few reasons.
1.) The plot for BTMRR was empty for the first few years of the park's existence, and was originally slated for the Western River Expedition, which thematically would have been quite different from Big Thunder.
2.) The original Big Thunder does not sit at the end of Disneyland's Frontierland. It's more at the heart. I realize this is a Magic Kingdom discussion, but if we're to believe there are deep layers of intent in the design of Magic Kingdom, I think the case is made stronger if we can find parallels in the park that inspired most of what's there.
3.) Even if this was an original intent at the time of BTMRR's construction, I think the "story" has been demolished by the addition of Splash Mountain. I have read statements from Imagineers that the original idea of walking from Liberty Square to Frontierland was supposed to parallel going west across the United States and simultaneously forward in time (from the revolutionary northeast to the expansion-era southwest), but Splash Mountain pretty much throws a huge roadblock into the idea, by taking you on a massive detour to the antebellum South.
Oh, trust me, I don't think this was the original intent of the land or Walt or anything else. I am just interpreting it as it exists in the current Magic Kingdom park. Was it the intent of the designer? Probably not. After all, how many times have we heard Casablanca referred to as a "happy accident."
Also, interesting point about Splash Mountain because although it is an enjoyable ride, it does little to add to the mystique that is Frontierland. The geographical and chronological journey takes a severe detour through the Laughing Place.
Admittedly it is a fun ride, but can anyone make a case for the inclusion of Splash Mountain on the aesthetics of a pretty historically accurate history of America?