Why should we assume corporate codes are morally just though? I do not. In fact I would argue this is evidence that company codes are in a general sense draconian and/or disconnected from social reality under capitalism, etc.
He's the CEO. It's _his_ code of conduct.
Many (most?) companies have specific policies against romantic relationships in the office, especially with subordinates.
It isn't necessarily about morality if there are rules in place that all employees, c-suite or otherwise, are expected to follow.
I don't know Nvidia internal policy, but assuming their HR policies include such a stipulation it shouldn't be treated as a "rules for thee, not me for" situation.