The Mirror Universe is some kind of true branch that happened much earlier, such that even the physics of light are slightly different.
Kovich’s explanation of branching universes suggests that they’re not common but also that as they increasingly diverge, it becomes less and less possible to corpse over and those who do may not be able to survive.
He noted that no contact had been possible with the Mirror Universe in several centuries.
The 24th century Kelvin Universe officer who came over to the Prime Universe in the 32nd century died, as would have Georgiou. But the Kelvin Universe was close enough that he was able to cross at all.