No, that's not the only explanation that makes sense - it's the only one you can understand. That's not enough reason to declare it "impossible". You haven't even said what it is that's "impossible". Time travel? Inter-dimensional travel (god I hate that phrase)? Putting letters in places?
Assuming a single timeline dynamic with (and then without) pre-destination effects, it's perfectly feasible to logically consider the possibilities - it doesn't have to be "right".
Assuming multiversal travel, pre-destination effects become less of a problem and more of a benefit to any explanation.