First, I could be totally wrong. I could swear I remember this, though, from one of the fairly few SW novels I've read.

Second, you have to keep in mind that most of the Star Wars fans out there never saw RotJ on the big screen, but only on video, and all of those video transfers have had the slugs. Even those of us who did see it on the big screen have seen it many more times, and obviously much more recently, on those same video transfers.

Third, if they did retcon those slugs into canon, then that would explain why they left them there in the Special Edition. Of course, my preferred reason for that is that they ran out of time and just didn't get to it.

I also seem to remember that they were there on the big screen Special Edition RotJ, and there wouldn't have been any telecine action there at all. Of course, there could be similar errors from when they scanned the entire film into the computer. I don't know if a telecine-like device would be used there or not.