IME, employees are on committees and boards (though not public company boards all that often) all the time. The issue here taking multiple full time positions. A CEO being the CEO of multiple companies at once is not common, and when it does happen it tends to draw a lot of scrutiny. CEO is considered a full time job, showing up to a board meeting every quarter is not.
The second part of this is disclosure, which was not done in this case.
"at the head" of large - especially publicly traded - companies is not the same as trying to run all aspects of the day-to-day. It also rarely (ever?) happens when they don't have a big ownership stake, or are there primarily as a figurehead.
We can debate if the executive timeline is too short and that's what destroys companies, but I don't see how this is the same as an over-employed engineer who's spread too thin.
The second part of this is disclosure, which was not done in this case.