They showed it at the DCFanDome Flash panel yesterday. They said about 85% of episode 20 had been shot and that is where most of that footage comes from.I'm assuming the footage in this trailer is from what was shot when it was intended to be episode 20 of season 6.
Maybe the shutdown will get the writers to tighten up their storylines and work in smaller chunks, giving more flexibility to pivot if the season gets cut short again. Season 5 was bad about having more episodes than story, and even though they tried to tighten up in season 6 by having two half-season arcs, there was still a bit of running in place going on.
Still, the trailer got me excited for the eventual return.