What should be established is whether the Borg could be created in the Mirror Universe.
Since the Borgs creation in our plane required a series of unfortunate events initiated by both chance and the personal decisions of several people (of which some will likely be dead in the MU),its nigh-impossible that the Borg could be created in the MU the same chance/convoluted way they were spawned here.
This brings me to my next hypothesis:
Isin't it more likely,then ,that the Borg in the Mirror Universe crossed over from our side ?
All it would take is one cube shunted into the MU by chance,and the rest would be history.