It may be better to put it like this: “if there are many worlds, then time travel would generally create loops across worlds; it does not force consistency within a single world”.
However, if the Source of Magic is careful how it sets up the loops, it can force a consistent outcome, or at least force one of the consistent outcomes to become much more probable than any inconsistent outcome (one which loops between worlds). In particular this still allows any NP problem, or indeed any PSPACE problem, to be solved in polynomial time using tricks like Harry’s factorisation attempt (though perhaps with a small probability of failure). See Scott Aaranson’s wonderful lecture here.
So the fact that Harry always observes a consistent single-world loop doesn’t by itself imply a single world interpretation, or any non-computability. It simply means that the Source of Magic is a PSPACE oracle!
It may be better to put it like this: “if there are many worlds, then time travel would generally create loops across worlds; it does not force consistency within a single world”.
However, if the Source of Magic is careful how it sets up the loops, it can force a consistent outcome, or at least force one of the consistent outcomes to become much more probable than any inconsistent outcome (one which loops between worlds). In particular this still allows any NP problem, or indeed any PSPACE problem, to be solved in polynomial time using tricks like Harry’s factorisation attempt (though perhaps with a small probability of failure). See Scott Aaranson’s wonderful lecture here.
So the fact that Harry always observes a consistent single-world loop doesn’t by itself imply a single world interpretation, or any non-computability. It simply means that the Source of Magic is a PSPACE oracle!