People like the men you mention were both robber barons and captains of industry. In a sense, the two were inseperable.
The men were captains of industry. It was through their efforts that huge companies were created. These companies made jobs for thousands of people. This makes them captains of industry.
At the same time, however, they were robber barons. We cannot deny that they did things that were very exploitative. We know, for example, that Pullman cut the pay of his workers while refusing to lower the rent that he charged them to live in company housing. We know that people like Rockefeller ruthlessly killed off (in a business sense) competitors and that Carnegie was willing to have strikers killed in a literal sense.
These men helped US industry and the US economy to grow, but they did so through unsavory ways. Therefore, they are both captains of industry and robber barons.