The article is titled "I Don't Believe in Full-Stack Engineering", yet states "If you’re only hiring full-stack engineers" as though they are something the author believes exist.
It seems that the real approach of this article should have been to outline why hiring ONLY full-stack engineers may not be a good idea.
It's a common idiom. Someone who says "I don't believe in psychics" means "people who call themselves psychics don't have the abilities that implies", not that nobody claims to be one.
It seems that the real approach of this article should have been to outline why hiring ONLY full-stack engineers may not be a good idea.