I have only seen discussion of separation masters existing for the original film. That's not exactly a completely standard thing to do for newer films, it's more common to just make a straight dupe negative. Seperation masters were used to repair SW, but there was never any mention anywhere of this process used on ESB, which to me always indicated that there is compelling reason to think that none exist. But then I am not the expert. Maybe Harris heard about their existance "through the grapevine", but as he has not had any direct contact or work with Lucasfilm or Fox regarding this issue it's really a bit of rumor and heresay and not something to just say "ah, don't worry, it's fine." I mean, I hope he is correct, if there is any guy to simply trust on his word it's him, but he is not being very specific here.
Anyway, he is right in a way, the existing duplicate materials aren't going to be unusable within the next 10-20 years and in theory they should yield very high quality results especially with modern tools, as there are lots of films that don't have original negatives. Personally, though, I believe it is doubtful how useable the negatives will be beyond the next ten to twenty years, just because they will continue to deteriorate, and that would be a crying shame because the films will never look as good as they do on the negative. Luckily the worst bits were repaired for 1997, but those original shots that were simply removed and put in a drawer somewhere will need a lot of work to bring back to life whenever the restoration comes, and even today the original negatives of them are not very useable because a lot of them are FX shots that were on the bad stock. If in 20 years the film gets a proper restoration, it will be very doable, but it will require a LOT of work to do right, and I mean a lot, and it's not clear that Lucasfilm would be willing to invest the money to do that, so the films are in danger of being struck from poorer materials with less effort put into them while the negatives continue to deteriorate and get worn out.