OK. I understand your question. You have to talk slow and use small words with me.
You are right. There are only three total scenarios. The one "What If" Argentina scenario allows you to play BOTH the actual battle and, if you choose, the "what if" scenario of reinforcing Mount Tumbledown rather than retreating. So, I guess, both Argentina situations are technically there.
But you are right that the manual is very misleading. Sorry for the confusion.
You could, however, very easily make the argentina what if scenario into a defense only scenario. You would just delete the reinforcements and change the victory conditions to reflect the lower number of argentinian troops. Just a thought if you are interested in tinkering with it.