A brief search on internet has showed me that, perhaps, the term 2D vs 3D for sound was used for gaming. Apart from the number of channels, either floor plane or heights, the 2D sound is when the sound is more static, flat, and does not follow the video scene (movement and or proximity). While 3D refers to sound changing according to proximity and/or movement of the video scene.
I may be wrong, but I have read something related to that.
Now, the immersive sound for music (beyond surround 5.1, 7.1) has been called 3D. And to differentiate from the 'only floor' speakers the old surround would be called 2D, like using a single floor plane level.
I think it does not matter where the terms come from, BUT that we all understand the same thing when we use the terms.