They didn't give a clear date. The language used was "expected release on or before Sept 21st 2016".... the qualifiers being "EXPECTED" and "ON or BEFORE" right off tell you there's no promised clear date. The most important phrase used is "expected"....as in "we are hoping, it should be ready by, etc."
If they wanted a clear date they would just put "World of Warcraft Legion releases September 21, 2016".... THEN you'd be correct on the clear day thing.
All that semantic stuff aside... based on what I'm reading from various alpha testers...unless they kick it up a notch and start really having killer productive weeks with killing bugs and adding content.... I really don't want Blizz rushing it to meet some select fans expectations of "well they said it'll be out Sept 21"....I want to play it too...but I'll wait an extra month or two if that mean it is polished (or as polished as a initial release can be).