I seem to recall some interview or Q&A or something around the time RitR was being announced, and its lower 200-pt format was discussed, that there was ways for things to occasionally expand your fleets. And the question of the SSD was brought up, with a suggestion that it could appear in some battles.
Now that we have it in-hand, obviously a few of the objectives do have that potential to have temporary allies joining your fleet to increase its size in that battle.
However...I don't see anything in the campaign that specifically allows for a Super Star Destroyer to show up. Unless I'm missing something?