7thCavalry

Definitely not. At some point during development Rockstar probably wanted the player to go to Mexico (but maybe ran out of time and we got a half-assed Guarma instead). This kinda correlates with Arthur at some point being able to return to New Austin (as proven by his unique dialogue lines and journal drawings if the players manages to glitch their way there) Anyhow, this is most likely just a development left over, and it's certainly more than just LOD and skyboxes as the area has unique weather cycles, ambient sounds and so on.