All those RG winners were not flukes- they were all excellent clay courters in an era of diverse surfaces. They couldn't win off it because it was too fast and they played with topspin.
But Verkerk or Gaudio diddn't have any great results even on clay besides that. I don't think there were flukes because there's no such thing, they diserved to win and reach the final. Cases like Costa maybe or Moya were different, because they don't have great results outside clay, but aside from their RG title they have some other achievements on clay.