It can be a weakness in excess, sure. But in a broader sense (beyond just athletes, but certainly including athletes), I think a certain amount of self-doubt is healthy. Not so much that it shuts you down or ruins your mental health. But enough to cause introspection - to keep you analyzing and working on yourself. Gets you back in the gym, or perhaps keeps you in it later.
I think more of him for saying this, not less. We all go through things. We should normalize talking about these dark places we sometimes go to. He went through it and came out the other side. That should be celebrated.
Perhaps this does have a 'monkey off the back' effect for him in the long run. But there's a new mountain to climb every year. So enjoy it, but there will come a time to get your ass back in the gym - that jumpshot looks pretty busted right now, lol.