This is the exact same type of "technical date" which we saw with Warlords of Draenor. Did everyone forget about that?
When pre-orders for Warlords of Draenor started in March 2014, the pre-order page said
on or before 12/20/2014. People flipped their shit because they didn't understand that it wasn't an actual release date, and Ion Hazzikostas
told everyone the reason for the date. Now that we're seeing another "on or before" release date, I think it's pretty safe to assume that the date selected has some sort of technical reasoning behind it, which is exactly why they state that it is not a release date.