Eh, beg to differ. There is certainly domain knowledge to be had, but the underlying principles behind any particular layer of the stack tend to be more similar than different across languages and frameworks. Everyone needs a database, authN & authZ, network requests, state management, pretty forms, error handling, optimized images, etc etc.

Knowing how these layers work as concepts is a lot more important than deep expertise with a particular implementation.

From the perspective of a new hire, you'll spend easily as much time learning their idioms, mutations of the platforms, unique glue code and business logic as you will learning the platform.