My guess is they are talking about some optimizations iOS does to make strings more performant, like interned string constants and tagged pointers. The majority of string usage probably falls under those.
Working with dynamically allocated strings, like reading from files, rendering from data bytes, working with C strings or building formatted strings, might have worse performance characteristics.
I think that's something the language does. I struggle to understand how iOS would do it.
iOS interns heavily across programming languages, and tagging and other small-string representations are available for non-C languages.