Technically aren't the tubes just pressed into the center housing? Folks have spun axles tubes.
Technically, yes, but there is a bit more to it than just pressed in. There is a finely calculated amount of press fit interference and then the casting is heated to expand the sides. When it is pressed in, there is also some very accurate alignment going on since both sides are welded up with the C and mounts before they go in. After they are in, then there are several plug welds to act as mechanical fasteners since the welds don't bond that well to the casting.