They always do this. From Wrath on, they show a little of the starting zone at Blizzcon. Then, sometime in the following late winter (think March or so), they release alpha or beta and that lasts 3-6 months, usually somewhere in the middle of that. A March beta that's 6 months long puts us at... (shock!)... September.
There's almost no way this slips out of 2020 just as there's no way it ships in spring. Come on people look at their history for releases.