If your departure from a company is a material fact by virtue of the position you hold, does it make it insider trading to dump your own shares in a company you are leaving?
Typically, if you're in a position where you'll often have material non-public info you need to create a trading plan in advance, or otherwise remove your ability to trade the company based on all the secret stuff you're always going to be knowing. Look at it that way and for the CTO of a large public co., their own departure is just going to be the last of many, many pieces of info make it problematic for them to trade the stock.