I would actually disagree with that statement. On a single unit to unit basis comparison - yes. Sure, you can throw an Alco 2-10-10-2 at a string of cars and it will outdo one diesel-electric locomotive by far. But steam locomotives can only be run in single units. Diesels can be (and usually are) assembled into coordinated multiple unit operations, and at that point they are essentially one unit in operation with any amount of power needed to pull any length of train.