There's an interesting question of "how much do the literal best humans suck at this?" For example, in chess Magnus Carlsen might be able to beat Stockfish given a handicap of just a pawn or two. An even better computer player than Stockfish might give up three or more pawns, but even a perfect player would likely lose to Carlsen if giving up a rook. -- I'm making this up, I don't think anyone knows the real values, but as far as I know no one is remotely projecting that perfect play could overcome e.g. a queen handicap.
Similarly, in Go it seems unlikely that perfect play could overcome a nine-stone handicap (again, I could be wrong, I'm not remotely a dan-level player).
All to say, it seems likely that Diplomacy is a game where the difference between "the best human play" and "the best possible play" is much larger than either Go or Chess.
We happened to talk about this at the Go club this evening. The strong chess players more or less agreed with you about the chess predictions, and the dan level Go players say AIs today can give the best pros definitely a 3 stone handicap (tried and tested), probably 4 or more, and perfect play is a few stones more (unclear how many, but probably not many, so not 9 stones altogether)
I attend the Ramat Gan Go Club, but there are Go clubs everywhere around the world, and they tend to be in the same places HN commenters live, go figure. See e.g. https://www.usgo.org/where-play-go
> All to say, it seems likely that Diplomacy is a game where the difference between "the best human play" and "the best possible play" is much larger than either Go or Chess.
Definitely, Diplomacy in general is substantially understudied compared to Go or Chess (largely because it's a tiny community). You can play for less than a year and get to top-level performance and much of the established wisdom/strategy of players is fairly bad.
Even the best Diplomacy players are only scratching the performance of how good someone could be.
Similarly, in Go it seems unlikely that perfect play could overcome a nine-stone handicap (again, I could be wrong, I'm not remotely a dan-level player).
All to say, it seems likely that Diplomacy is a game where the difference between "the best human play" and "the best possible play" is much larger than either Go or Chess.