They might as well have said that the swords are real heavy too and that would have slowed them down as well as the heavy armor. But like you said, the people wearing them trained in them, so the armor wouldn’t have been as much of a problem for them as four volunteers who never wore armor in their lives before the experiment.
If your arm is twice as heavy for being encased in metal then you can slash your sword half as often regardless of how fit you are.
Analysis of medieval knights’ skeletons shows that the bones of their sword arms and shoulders are much thicker & heavier than the other, probably due to the extensive training.