In that episode, Captain Janeway (in the alternate timeline), spent another 16 years in the Delta Quadrant after their encounter with the Borg in the nebula.
In that timeline,how did Voyager eventually make it back to Earth? Secondly, they developed weapons and technology such as deployable ablative armour and transphasic torps yet in the 16 years the Borg couldn't find ways to adapt?
The details of how they get home are never covered except in vague generalities. They only tell you enough to know that they traveled for many more years, lost some friends, gained some new friends, developed, found and implemented new technologies, and eventually found a shortcut or two home. It's never mentioned when those new technologies were developed. Maybe they only had ablative armor for a couple years.
As for the Borg, they are only able to adapt to technology that they are able to assimilate knowledge about. They lack creativity to imagine new technologies and there are races advanced enough to prevent assimilation, unless the Borg assimilate some technology elsewhere to give them an advantage. If the Borg are never able to assimilate someone from Voyager that has knowledge of the technology or someone who understands another technology that would allow them to bypass those techs, they would not be able to adapt.
At their starting point beginning, Voyager had already travelled approx 500 lightyears in it's first year in the Delta quadrant, Kes's contribution shaved 9500 additional lightyears, their experimenting with slipstream took off 300 light years, the subspace wormhole in the Void shaved 2,500 lightyears, once again a refined slipstream system shaved 10,000 lightyears, stealing the transwarp coil and it's use cut 20000 lightyears. The Subspace corridor (ep: dragons teeth) cut 200 and the space catapult shaved off 600 more. By then they were 30,000 lightyears from Earth. In the 16 years it took, Federation/Starfleet may have already developed enhanced propulsion technologies, tech which could have sent to Voyager as communications. By season 7 they had established permanent long range communications with Voyager, presumably permitting Starfleet to send specifications for Voyager to upgrade it's propulsion systems that would have shortened it's journey, not to mention the potential stellar phenomena and other potential transit solutions that could have shortened their journey.
