Maybe your body is digested over a thousand years because the Sarlaac doesn’t catch very much prey so it has evolved a hyper-efficient digestive system and sits dormant most of the time, but also once you’re in Luke’s right and you’ll actually die of crushing or acid burning or dehydration.

But they also call it “The Almighty Sarlaac”, which to me implies that this is all folk knowledge. So maybe “be digested over a thousand years” is just what Tatooinians believe will happen to anyone who falls in.