But they are 90’s in 4 dimensions. And the interior cube is the same size.
As an analogy, think about drawing a cube on a piece of paper.
The angles on the paper are not rt angles, but in three dims they would be.
The cube angles are right angles. But the diagonal supports going from one corner of the outer cube to the corresponding corner of the inner cube are not. My remark is about the angles on the supports.
The inner cube, based on the drawing, cannot be the same size as the outer. The inner’s 6 walls are presented as being inside the outer cube’s walls. For perspective to be in play, at least one wall would have to be outside the outer cube.