Quotes should be taken with a grain of salt. They certainly shouldn't be treated as concrete facts, seeing as they are usually contradicted by what actually happens in the story.
Now I can buy Vader holding back somewhat since he wanted Luke alive, but keep in mind that Vader is a Sith. Their typical mentality is "if you're that weak that I can easily kill you when I'm barely trying then you're not worth my time."
My point is, Vader wasn't holding back "that" much. He certainly didn't let himself get booted off a ledge or let Luke tag his shoulder.