Shield Sphere has the Wall subtype, but that is mechanically meaningless. All Walls (that are this type on the typeline, not changelings, etc) now have the keyword defender, which means they can't attack.
Does the text "Counts as a wall" on Shield Sphere for example mean that it IS a wall and therefore cannot attack (under normal circumstances)?
No card has the text "Counts as a wall". Every card has the text of the Oracle reference for the entry with the given card name. The print on the object in your hand is there because of inertial properties of matter, only.