That was definitely not their business model. For almost a decade, I would rarely keep a disc for more than a day. I would be well into the 'Heavy User' category. Netflix would receive my disc, send out my new movie the next day, and the day after that I would have the new movie. My father made the same claim as you about 'throttling'. When I looked into it for him, it turned out that even though he lived in a major metropolitan area, his mail was making an extra hop to a sub post office before delivery. This added both 1 day extra for Netflix to receive the disc, and an extra day for him to get a return disc.
I don't believe that throttling has ever happened at Netflix. People just became convinced it was happening to them because Netflix would give low volume users first pick of movies. Everyone would get a movie shipped out, but if a heavy user and light user wanted the same movie, the light user would get the movie, and the heavy user would get the second movie in their queue. This is not throttling, but lots of heavy users convinced themselves that it was.