The point wasn't whether Blizzard wrote it or not. Of course it is their game and they can do what they want. So that too wasn't this issue.
Whether or not this change is a contradiction of the lore is the issue. Arm waving about Blizzard doing what they want isn't going to change that.
The lore is that the Universe of Azeroth has primal forces at work across various dimensions.
Death, life, light, dark, the void and so forth. All of these forces work in multiple dimensions.
With the introduction of Shadowlands it was made clear that death related magic and abilities originated in that dimension.
They also expanded on previous lore claiming that demons got their death powers from there. (I didn't agree with that but that is what they said is the lore). So that means death related powers work outside the shadowlands in the lore.
The issue is what is a covenant and where does that power come from?
Why should you be given arcane abilities from within the realm of the dead in the first place?
Arcane isn't a "death power". Neither are void, shadow, frost and fire abilities and so forth.
These other powers work in the shadowlands just fine just like living bodies somehow work there as well.
If these were only death powers unique to the shadowlands that would be one thing, but they aren't.
These are class/spec abilities that have nothing to do with the realm of death to begin with.
So technically all of this is a house of cards lore wise no matter how you look at it.
If shadowlands powers don't work in the living realm then why do powers from the living realm and other realms work there?
Sure Blizzard can do what they want but that doesn't mean it makes sense.