Unless you are going up a very steep hill, most any wheel barrow is easy enough to put down to a rest. And combined with a shovel, I use barrows far more than just trying to manage a small bag of rocks in the yard.
Granted, if you can just throw more people at the problem, I fully agree that a team of people will be faster than a single person with a wheel barrow. But, that has more to do with it being a team than the barrow.
Well this area is in the Andean mountains which is pretty much as steep as it gets. The only real flat land they had was the strip of shore on the coast where boats would be better methods of travel and transport.
This seems to imply that none of the rest of the continent would have been viable for travel? It isn't like there were no wheels in mountainous parts of other continents.
Granted, if you can just throw more people at the problem, I fully agree that a team of people will be faster than a single person with a wheel barrow. But, that has more to do with it being a team than the barrow.