This is correct, but for Harry to regard the claim “they will live beyond today” as absolutely true (or as close to absolutely true as you can reasonably get), he has to both (a) have no intention of killing them at the time of making the statement and (b) not anticipate that intention changing over the course of the next twenty-four hours or so.
Notice that in my latest suggestion, I phrased it as:
“If you and your servantss leave girl-child and me alone here and causse uss no harm before or in doing sso, it iss very likely that the world will continue to exisst in more-or-lesss itss current form for the foresseeable future and you will all live passt thiss day.”
That is indeed a good way of hedging your bets, and I agree that it works very well in the context of the world ending. However, I’m not sure Harry can even call it “very likely” that the Death Eaters won’t die within the day; the probability that someone ends up killing them is nontrivial. (Of course, I’m aware that your suggestion doesn’t include anything about the Death Eaters’ well-being; I’m just talking about the statement I originally suggested, more for the sake of argument than anything else.)
Notice that in my latest suggestion, I phrased it as:
That is indeed a good way of hedging your bets, and I agree that it works very well in the context of the world ending. However, I’m not sure Harry can even call it “very likely” that the Death Eaters won’t die within the day; the probability that someone ends up killing them is nontrivial. (Of course, I’m aware that your suggestion doesn’t include anything about the Death Eaters’ well-being; I’m just talking about the statement I originally suggested, more for the sake of argument than anything else.)