Cameron and crew have started “concurrent” production of the sequels, which are poised to make cases for both high frame rate video as well as Avatar‘s signature blend of CG with real-world acting. In theory, this gives the team a better sense of the timing than it might have if it was taking a serial approach.
With that said, you may still want to take these dates with a grain of salt. It’s not just that the releases have been pushed back in the past, it’s that the scope has changed over time. Cameron added a fourth sequel to the mix just in 2016, so it won’t be surprising if the schedule shifts due to further creative changes or unforeseen challenges. Really, the big news is simply that the director is getting the ball rolling after years of prep — the dates just give you a rough idea of what to expect.