Selling shares to raise money to donate to charity feels highly likely to net negative social good regardless of how the funds are spent.

You could simply retain ownership of an important biomedical company or you could share it with employees.

Selling shares for money that you then give away is dumb.

Selling shares for money to donate to charity is actually a tax mistake. I've done it and regret it. Just donate the shares to the charity and let them cash it out.

Surely the CEO has a tax advisor to advise... so this leads me to believe that very little of it will "go to the charities"

It wouldn't always be a mistake. Moderna stock is significantly lower than it was in 2021 so the CEO could be harvesting losses.

Pre-planned sales for execs are often buybacks or are sold to selected investors/shareholders.

Companies often impose limits on how many shares execs and employees in general can hold at any give. time, and most regulatory/legal frameworks put caps too.

In the UK for example when employees own more than 5% share in a company they are no longer treated as regular employees.