Dumbledore isn't in the mirror. Harry and Voldemort are. The trap triggered, stuck them in a time-dilated mirror plane. Which is, of course, the only reasonable way to deal with an immortal serial killer. This even counts as fulfilling the darn prophecy, doesn't it? Definitely if Dumbledore can extract Harry, because that way naught but a remnant will exist in the same world, and yes, it does count as Harry defeating him. He talked him into stepping into that trap, after all. Bravo.