I think the ion storm provided enough energy to allow a transporter to bridge the gap between the two universes. As the storm effect fades, the transporter will need more and more energy to bridge the gap. Once the storm effect fades to a certain point, the Enterprise cannot generate enough energy to bridge the gap. It sounds like the mirror universe was already there.
SCOTT: The two-way matter transmission affected the local field density between the universes, and it's increasing. We've got to move fast.
KIRK: How fast?
SCOTT: Half hour at the most.
KIRK: If we miss?
SCOTT [OC]: We couldn't get out of here in a century.
...
SCOTT: Hardly fifteen minutes, sir. The field density between the two universes is starting to close very fast.
I'm not thinking the theory is correct at all but I think the schism between the universes probably happened much later than what we've been shown in the series! TOS and DS9 MU mention the events from the first episode so they're the same Mirror Universe where as ENT and DSC Mirror realities must be other ones as they don't actually fit in with established events and since DSC and ENT aren't the Prime Universe anyway...

JB