You know, being the CEO of this company, he know his vaccine better than most of the people, but that doesn't mean he know how much fiat money the FED can print to support your fiat stock market as well
So his selling decision is made based on the best knowledge he know (e.g. how good the vaccine they developed).
In China, his case is still considered insider-trading, even if he loss money, but I know in the wild-west american his action is perfectly legal, actually more legal than the senators who proved they were better at market timing than even the most talented professional traders.
On a side note, I know some guy who just resign from Pfizer recently, he believe the vaccine they developed is a disaster waiting to happen so he decided to jump the sinking ship.