He's likely referencing that you would need to use different syntax and style, like re-assigning a variable or chaining calls, like when working with a String in Java.

In C, you can simply mutate the underlying characters. So changing the fifth character in a string is as easy as:

    str[4] = 0;

Whereas using the immutable syntax you might have to do something like:

   str = str.substr(0, 4) + "\0" + str.substr(4);