I guess he could have had the deathworm in him the whole time since then. He obviously didn't completely die in the meantime, and I don't think there's any suggestions that he switched bodies at any point since then, just did a lot of jiggery-pokery with the one he had.
That last bit does seem to suggest that he went right to Skaro, though, which is a nightmare to reconcile with the VNAs. Wow, that was salty of them; writing off their continuity in the very first novel.