I would disagree, when you speak with people, you use different parts of the brain than When your talking and no one is around. this part is also used when singing so that’s why you don’t stutter when singing. (Although they are both on the left hemisphere of the brain). A speech block is a speech block and it’s due to some mechanics in the brain which are not working properly, you can’t blame it all on anxiety etc, as I know a few people who don’t suffer from anxiety but still produce speech blocks