Austria is also the player instigating a plan of action in the dialogue, which seems to be how the AI is so effective. It seems like the way it wins because it proposes mutually beneficial plans and then (mostly) follows through on them.
Is that also useful for human players to do? (I have never played Diplomacy.) That is, in negotiations with other players, be the first to propose a plan and so set the agenda for the conversation.
Yes. From page 34 of Supplementary Materials:
In practice, we observe that expert players tend to be very active in communicating, whereas those less experienced miss many opportunities to send messages to coordinate: the top 5% rated players sent almost 2.5 times as many messages per turn as the average in the WebDiplomacy dataset.