I guess you're right that it could be partially between the two, but it seems like current cannon is wording away from that idea with the explicit
Illidan book quote. Netherstorm would still make sense in that sense, though, given that it's highly inundated with energy. I think the counter point could also be made that the Broken Shore probably has more portals to other areas in the Great Dark and the Twisting Nether than all of Outland combined, but it doesn't appear to behave by the same rule.
I'm not going to outright disagree as there are a lot of people that hold to this, but some thoughts:
1. Isn't Azeroth only visible in the Black Temple skybox?
2. If Outland is in the Twisting Nether, certain key demons slayed there during TBC should be dead, but they appear later. Socrethar comes to mind.
3. On alternate pervading argument I seem to read a lot is that people are saying Azeroth is only viewable from the Black Temple because Illidan is scrying on the planet as he tragically ponders his decisions. Kosak
seems to support this idea after
hearting a Twitter response from 2015 that says this very thing. (But he doesn't say it himself, and earlier in the replies says that the Twisting Nether is the space between worlds, not the GBB, so either he's confused or the retcon about the two wasn't established yet then.)
I guess it's just really convoluted if anything. I think the bigger issue was that for TBC--which had hardly any actual storytelling in it--was largely just a big clump of "cool ideas" compounded by people with little actual storytelling or writing experience, and that now as they try to streamline the lore and rules of this large, complicated universe, there will always be loopholes and oversights because of the nature of how older content was developed.