they kinda sorta cheated imo, I watch a lot of top-level diplomacy gameplay and listen to analysis, and the facebook ai was in a blind version with only a minute or so between phases, leaving hardly any time for actual negotiation. It also made use of a lot of human shorthand move codes built for these blind blitz games to simplify it's communication. The mode it played also had some other changes like removing the winter build phase as a time for negotiating. The "normal" version of the game has multiple days between phases, and people write many paragraphs to one another.

It's still pretty cool, but its not like it just convinced people using raw charisma. Yet.