I think there’s a couple reasons they do it this way.

One is that the pre-order bonus is still available despite the game effectively being out. I imagine they spare themselves some unwanted difficulty or dissatisfied responses from people who otherwise would have missed it.

The other is this very thread. Server issues are common on an expansion pack release. This gives them a convenient excuse to put in the apology announcement. It’s a small thing but who knows, maybe it has some impact.

It’s definitely a silly twisting of words (and their double key system for the pre-order and full purchase only sillier).