If I may throw my hat in the ring, I think it makes more sense for the Source Wall to emanate in a lower dimensional form in lower dimensional planes.
While I do get the point of the Infinity example, that's different. If you take a finite amount off of an Infinite 3D wall, yeah, that would be finite 3D. But, this is relative to the dimensionality of what you're affecting, for example, if you took a finite amount off of an Infinite 4D wall, that'd be finite 4D. In this case, where he takes a finite amount off of a wall beyond dimensions, he's still affecting something far beyond 3 dimensions. I also don't get how a wall that's beyond dimensions can exist in a 3D space, so it just makes more sense for the wall to emanate in lower forms imo.