Given the timing, it certainly seems like that could be the case, but at this point I could see the delay being the result of even more fine-tuning before production begins. Perhaps there was some consideration over whether to delay it again, and then Disney decided to
Star Wars the crap out of the next two Decembers and that just made up the minds of Cameron & co. for them. Then again, it's not like Disney has anything to gain from making the sequel be delayed too long given that they're adding this world to one of their parks...
What interesting things are being discussed in the Reddit thread? I'd look myself but I typically can't stand any kind of online discussion of
Avatar outside of the main forums.