Kevin Feige (President of Marvel Studios) has verified that all the new Netflix Marvel shows are part of the the Marvel Cinematic Universe (which is what Marvel Studios is calling the shared universe that all of their recent movies and TV shows takes place within). So this would be a reference to the Battle of New York. If you've seen the Avengers movie, it's the huge battle at the end where Loki and the Chitauri battle the Avengers and S.H.I.E.L.D.
Ultimately, I think that Marvel is banking on the fact that most viewers of these properties will have seen (at least some of) the other properties in the MCU or will be familiar with the comics they are based on.
Also, in this particular case, what is necessary to know is that the city is rebuilding after a disaster. Whether it is 9/11 or the "Battle of New York" is a detail that doesn't strongly impact the story one way or another.