My understanding is that, given Tesla is a public company, any collaboration between Tesla and other Musk companies needs to be approved by the board (or maybe by other C-suite executives) without Musk in the room. Musk can come up with the idea for a collaboration, but then the decision that it is in the best interests of Tesla and all its shareholders to proceed with the collaboration needs to be made independently of him.
By contrast, I don't think the rules apply as strictly to his other companies since they are privately held. The law cares a lot more about protecting shareholder rights in public companies than in private ones.