I guess that, technically, there's a hole in the rules in that it doesn't say what would happen to the Tarrasque if something would kill it instantly without a save. We know that it doesn't kill it instantly, because it's not Wish or Miracle and those are the only things that can kill it. But the rule about it taking 868 nonlethal damage only explicitly applies to failing a save. So a DM could rule that a Sphere of Annihilation or drowning just does nothing at all to the Tarrasque, I suppose. But it's probably more reasonable to assume that those things have the same effect as insta-kills that do offer saves.