The most logical way of looking at it? Hard disagree. Four people from the Mirror Universe beamed aboard the USS Enterprise when the landing party crossed over and beamed aboard the ISS Enterprise. It wasn't just some daydream that the landing party experienced.
I didn’t say it was. I said it was the most logical way to interpret the episode, because there’s no way that the PU and the MU would be anything alike at that time with what we learned about it later.
And those four crewmembers? An illusion too. We have no idea of what the Halkans were capable of, just like we had no idea what the Organians were capable of until they showed their true colors.
Of course this is all science fiction, so one can interpret things any way they want. Obviously this wasn’t the “Mirror Mirror” writers’ intent, or the intent of everything that came after, but one can certainly argue the case for their own interpretation.