They're not, actually. They're a part of Dawn Treader, and the Lone Islands episode is somewhat larger, with a more direct confrontation between Caspian and the corrupt Governor Gumpas after Drinian and co. free them from the slavers.
My apologies. It's been a long time since I've read either book, and I was misinterpreting what Apted said about pulling elements of
The Silver Chair into the film of
Dawn Treader. I thought it was the slavers that came from
Silver Chair, but it appears that it's the green mist that has some connection to the latter book. And some Narnia fansites say even that's an exaggeration, so I don't know what to think. I should just reread the books.