Full-stack is not a disparate set of tools without expertise. Full-stack means making complete, functioning things. From start to the finish. The specialization is being able to take something from nothing and bring it to become something that actually works.

Even with that, specializations for full-stack developers reach ~80% in areas which their specific work requires. This may be backend, it may be front end, databases or anything that is heavily used in that particular sector in which the developer works.