Selling everything = I have no faith
Keeping everything when these shares represent almost your entire net worth = I know with 100% certainty that this business will grow
Selling off 25% = I believe in the company, but I also acknowledge that there are many variables outside of my control that can affect the success of this company and I don't want those to have huge negative repercussions on my life.
Both his past and present takes are perfectly reasonable. If you don't know enough about something, then you don't know. Simple as that. Much better than being confidently incorrect (or even confidently correct) about it when you don't actually know the topic.