Earlier quoted context omitted.
There's also the obvious conflict of interest. How does that get resolved?
Is this a moral or legal code that is being broken somehow? Because he owns both companies it is somehow immoral to do business between them?
He largely owns X which is a private entity. He owns ~20% of Tesla, and the rest is owned by other shareholders. As a company officer he has a fiduciary duty to act in the interests of all Tesla shareholders, and any sales or deals to other entities he has in an interest in pose a conflict. That conflict can often be mitigated by oversight or deferral of decision making to parties that don't hold the conflict such as other board members or corporate officers.