Others have mentioned how the 3D realm isn't truly 3D because the character is still largely moving only in 2D; I think this fits perfectly with the fact that thinking outside of the dimension you exist within is incredibly difficult. (Try imagining a 5th dimension and the logics therein.) The character dreamt of forward and back, sacrificing up and down, and still in both existences they are trapped in one fact: there is no goal, no 'point' that they aim for. Their dissatisfaction with their reality traps them in a new, discomforting realm, with the same vacuum of purpose staring from the meta.
This game is good at inciting a spiral of analysis because I could go on.
And of course, the inbetween, where you navigate the 3D maze with the 2D platformer is *chef's kiss*