So in short, he did NOT go back to the Earth where he changed things around, he went back to ANOTHER timeline where he still got de-Thingified, and maybe even by an alternate Thing going back in time to do a favor to a him from the past, but there were other factors that were different which had nothing to do with his past time-trip. That would make sense, I guess - I mean, time travel ain't just a walk down the street; it must be tricky to return to precisely the right timeline at the right time and place.