We can confirm that the Borg were very much in Federation space long before the their attack on earth in TNG BOBW.
Who says El-Auria was in Federation space?
According to the novelization and an onscreen graphic, one of the ships bringing El-Aurian refugees to Earth was the SS Robert Fox
(named for an ambassador featured in TOS), so it and the SS Lakul
were perhaps Federation ships, not El-Aurian, but the Borg may have attacked and scattered Guinan's people many decades earlier, and a small group of survivors was finally making its way to a planet Guinan had visited before.