Dear WW: if a 150 lb firewman climbed 900 feet in 720 seconds, he had done 135,000 foot pounds of work in 720 seconds, or 187.5 foot pounds per second. 187.5 divided by 550 equals .34 horsepower.
Of course the Indians carrying 200 pounds almost certainly had to rest a couple times on the way up. But they did it twelve times a day, so it was still remarkable.