Of course that is not applicable to 'boards' and 'CxO'. Quick search suggest some of them sitting at at least 2 'boards' or also 'partners' or something.
So yeah, rules for thee but not for me.
The difference here is that as an employee you commit to provide 40 hours of service to your employer. From the stories I’ve heard of these double jobs, this isn’t a weekend job working at the local coffee shop for an extra few bucks which doesn’t impact your primary responsibilities. Instead it’s a professional job with overlapping hours. Which means an employer isn’t getting what they paid for and these people are deceiving them.
Most CxO positions are full-time although there are, e.g. fractional CFO-type positions for smaller companies.