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.
One particular CEO in the news is at the head of 3 companies
"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.
Many would argue that, indeed, said public CEO is spread too thin.