And yes it is a prequel, but you know what they mean by sequel, it was the fourth movie released in the series, so it is a sequel in the way it was released, but it is a prequel in the way it is in the story.
There was a list a couple of months ago in some magazine that had the top scifi movies ever and ESB was #1 and Star Wars was #4, ROTJ was nowhere to be found in the top 10. That list was interesting to see how the original SW and ESB are still the favorites and have aged very nicely among scifi/fantasy movies.
It is just a list, not the law.