That line really made no sense as the Borg had clearly NOT assimilated whole Federation worlds or forced them to fall back by the time of FC.
That line is the entirety of the information we have on the subject of whether or not they had done so. And it says they did. So, clearly, they did.
And don't think I didn't notice that you dodged my other answer. In Way of the Warrior, the Defiant finds victims and passes by them because it would be too risky to try to help; in other words, they "decided to leave them to their fates because they were scared".
That clearly matches what you asked for.
It's one of several examples where the heroes either fail to help someone, or decide helping would be too risky.