I've never heard "She affirms so" in English.
Even "She affirms" or "She affirms, yes" would be strange.
We generally say, "She confirms" to repeat an intention to do something or agree to something.
To affirm usually means "to announce agreement with a belief or statement". So, "She affirms her believe in" or "She affirms she will do sth" would be more normal.
Mind you, "She so affirms" is proper English - but it is both technical - as in Court of Law, and archaic. I agree that it's unusual and we would normally say, "confirms", but in the US forces, people usually say " affirmative" instead of "yes" as a way of making the message clear. Remember that Duo uses US "English" as its default setting.