I'm not clear on that. My reading on the extremely tentative, "Blue Sky" plans is that this is literally what one would see if one was a traveler and walked a bit
past the town in which Thunder Mountain is located. Which doesn't make a lot of sense, because Mexico and South America clearly wouldn't be in the same place, even if they want to say it's a border town overrun by villains for some reason. I guess they could say that Thunder Mountain is a gateway into the realm of fantasy for whatever reason, but again, that goes back to what I said in my comment above. Why wouldn't it be a gateway into
all fantasy, including Cinderella, Mickey, Ariel, etc.?