Do you think Alphinaud should have brought up that they had the definitive upper hand in these negotiations?
What you might think he sounds like: "If we wanted to invade and use violence, we could have, and the fact we haven't is testament to our intentions."
What the Legatus will hear: "If we wanted to, the Warrior of Light here could kill you, your guards, and every man, woman, and child in a ten mile radius without breaking a sweat. Now, do what we say before we unchain this savage monster on you all."
Your average Garlean citizen is, as Alphinaud learned earlier when three malnourished men stabbed him behind the shed so a defenseless woman can drag her sick sister across monster-infested frozen wastelands just to get some physical distance from us, pretty untrusting of Eorzeans. What do you think a Legatus -- especially the one who was fanatically serving the Empire after the Emperor's demise -- would think?