When I see things like this, it makes me wish I had the ability to just "spin up a theater" just to run one and call their bluff. Perhaps this is just naive bluster, but I there are few things that make me stand my ground like threats.
If you start thinking about the logistics, odds are, this is a very remote possibility.
Most of the reports have said the hackers are either in N. Korea or Eastern Europe. They've threatened US theaters here. So either this is a huge bluff, or they have some serious sleeper cells here in the US they are going to activate them and have them attack theaters in specific cities where the movie is being shown.
The problem with North Korean "sleeper cells" that are resident in the US, is well...once they're here, they kinda shift their opinion on how the world works.