You're 100% correct. Only 2016 was ever announced. I'm sure the heads of the project are striving for a certain time of the year, but I bet even THEY don't know when it'll be totally ready/finished by. So it's definitely been assumptions based on the progress so far/previous openings. But I think by Summer 2016 is a valid guess. Have it open in time for the summer crowds.
Oh my god. I probably won't enjoy the Wizarding World until 2018....