It is, under rule 10b5-1. But so what? The CEO goes through with a sale of stock that was planned and announced months ago, at a time when he definitively had no material non-public information, and everyone goes shocked Pikachu face? It makes no sense.
Having to plan it months in advance takes some of the short term trading aspect away. You can have no idea what markets will be doing months from now.
You obviously have material non public information as CEO, but you could just as easily lose by trying to trade that as win when there’s such a long delay involved. Everybody else gets to see what your trade is, with plenty of notice, and can front run you.