The south America of Earth 18 civs is a late-game powerhouse, not only because of grassland, but mainly because of most of that grassland is riverside one.... add US and a levee and you have 2 rawper tile along with the town money.... Adding to the fact that most of south america is below 30º ( Space Elevator ), who controls South america is in a prime position for a launch.....
All that is true, but it's not necessarily of any use to Huyana. It's pretty easy for one of the North American civs to settle the land first, and sometimes when I've played I've seen South America end the game as an unco-ordinated mix of barbarian cities and European colonies.
Basically, we can win with Huyana, but it's still almost certainly the hardest start on the map.