An insignificant space is only really "Infinite" from the perspective of a normal lower-dimensional being
The problem itself doesn't even exist, because you're assuming that "Infinite" would not be referring to the subspace itself, when that's not the case.
It's the subspace being called infinite, not something else
To top that off, we know that ramiris spans her labyrinth inside subspace; the Labyrinth extends in that subspace. However, the normal world residences don't perceive it as going "underground" or "in the sky". Why? Because its extension in the 3rd dimension with respect to those beings is 0.
Take it as this; if X, Y and Z are the 3 normal dimensions, and U is the 4th spatial dimension, then normal people can only perceive something that is not only within 3D space, but simultaneously also has non-0 extensions in all of those 3D Spaces.
On the other hand, that's not the case for Lab, because Labyrnth's spatial extension is 0 in one of the 3 normal dimensions [X, Y, Z], instead, that aspect is extended in the 4th dimension, U.
So the Labyrinth's coordinates would be like [1, 1, 0, 1] where the last one is always the 4th dimension U.
Yet the infinite corridor is infinite in comparison to the labyrinth, which sufficiently speaks for itself that the "Infinite" is referring to the extensions of U[4th dimension] and other 2 dimensions[let's say X and Y].