I have seen that floating around. Is there any reason why this is being so? They are just two different companies belong mostly to the same person.
Why would owning Tesla gets your SpaceX?
It doesn't, at all. I've heard suggestions that great news coming out of SpaceX does impact Tesla, but not the other way around. Plus, SpaceX isn't a public company.
And they would have to pay market rates for that. There's no upside there.