I think he's saying stuttering is made worse by anxiety rather than caused by it. And in most cases, I think that's probably true. But not always of course. Still, I don't think it's ever really bad advice to encourage people to stop worrying about what others think of them. Also nobody really knows what stuttering is or what causes it. So when you say it's biological, it's no different than someone else saying it's psychological. Maybe one, maybe both, maybe neither are true. Nobody actually knows.