Your point would be clearer if you set "anders" in bold in place of "iets". That being said, although I agree that there is a difference in nuance, where this translation indicates that the thing being said is different from the things being said before, I still think that it is a valid translation. If the speaker puts a lot of verbal emphasis on "nog", then that emphasis would indicate that the thing being said is different as well.
This is one of those Dutch sentences where I find difficulty in remembering which English translations are acceptable. I have got it right before, but not this time. I said, "I want to say one thing more", and it was marked wrong. Granted, the emphasis is on the word "more", but the meaning is the same: I want to add something to what has been said already".