These are two different concepts. They’re both useful ones. Interpreting an argument charitably has positive arguments in its own right. It can promote healthier conversational styles and more effective reasoning. Actually understanding what your counterpart is trying to say is difficult, and that is why Intellectual Turing Tests are non-trivial to pass and valuable to engage in.
Steelmanning is useful for changing your own mind, but it is at best rude to the person you are in conversation with, and can as a practice close off more substantial shifts in belief. The argument most likely to change my mind is going to tend to be the one that produces the smallest update, since that needs to conflict with fewer other beliefs about the world.
We should keep both terms, not crush the principle of charity entirely.
I have made a good-faith post as a result of the extension!