I had a look back through the timeline here, and 92 days isn't necessarily too short a time in care, especially since a month before his release he was able to fly 50ish feet horizontally in his flight cage and being able to fly/launch vertically to the perch where he's sitting in the cage is even more difficult and taxing on the wing. Had the bird been kept over the winter, he wouldn't necessarily get any better, just more practised over short distances/heights. Keeping him past the migration period probably would have meant holding him until just before spring migration - so another 5 months. I know Kitchener has prey all winter long but it is very likely that he would have headed south and would have had to rely on whatever prey he could find en-route - not so easy in the winter. If he had stayed in the area he would have had to get back up to speed hunting in an urban environment, which is pretty challenging even though there is a a consistent food source.
It sucks that Lucifer was lost particularly as he was young and seemingly doing well, but it is entirely possible that it was just an accident, one that could have happened to any peregrine - recovering from injury or not.
Of course, I don't know the bird or the rehabbers, so these are just my thoughts.
