^That would've been a rather absurd coincidence, to just stumble across a ship exactly like the one they'd previously captured.
Unfortunately I can't think of a good alternative explanation for how they would've gotten the BoP back. Logically, after the Mirror Universe Saga, Starfleet would've taken the BoP into its possession and taken it apart to study Klingon technology. They wouldn't have just let it sit in the Excelsior
's shuttlebay even if it were small enough to fit there. Having it be intact and functional at all would've been hard to justify.