How was this determined? Online play?

Yes from online play but there aren't any official rankings. It's hard to determine exact rankings because it's rare for 2 world class players to face off over a large enough number of hands to give a significant sample but a loose hierarchy does exist. You can tell where a player sits in that hierarchy by who he is willing or unwilling to play. It's not an exact science but I agree that all of these players are unquestionably elite and the AI has beaten them to an extent that it would be a very strong favourite over a team compromising of the true top 4 players.