Well, what do you think?
I try not to over-think mirror universe or time travel episodes (or movies). There are all sorts of questions and problems that come to mind if you think about the "how." Usually, these episodes are either about a great story or great fun. How they got there and back, how everything was put back into place, how nothing ended up changing the future, etc. -- yeah, the time travel stories are better for me if I don't ask those questions. Haha!