If you already have all the models in the army of any of these armies the boxes probably aren’t aimed at you. That’s hardly specific to the Nighthaunt one.
I’m not saying it doesn’t make sense, just that you’re making assumptions about how GW view it. They might not be worried about having them all out by Christmas, or they might have a different deadline if they have one at all.
But the whole ‘hitting Christmas with all of AoS having battletomes’ Is just something you’ve said about. If anything, I imagine hitting a new financial year with all of AoS having updated battletomes is possibly a bigger deal to GW, if they even see it as a deadline like that.
They said at WarhammerFest there was still at least one tome for each GA to come this year, and that was after Slaanesh came out. That might have changed by now, of course, as that was quite a while ago.