Given the medical/biological explanation for the "darkseekers," I can see no plausible explanation for the extremely rapid burning (with something like smoke) of the creatures in sunlight. This is much more like what happens to supernatural vampires in sunlight.
The only guess I can come up with is that their paleness means they have no protection and they also have very fast metabolisms which accelerates sun burn but that is not really a very chemically plausible explanation.
I do not recall what effect sunlight had on vampires in the novel and if they indeed burnt in sun, how this was accounted for.