I wonder if the players tried colluding: coordinate their bets to fake a weakness so the AI start to adopt a poor strategy, then up the bets and stop the feign. I don't see how the AI can protect itself against that.
As to your point about the EV, this is why collusion can work. By colluding over a long enough horizon, the AI can believe that the average expected value to be something that it is not. If only one individual feign a weakness and the rest do not, then the strategy doesn't work.
For example attempting to feign weakness by betting small in a spot where your entire range should bet large is not tricking the AI, it's just passing up on EV for the players, good players are not going to play poorly in hope of tricking the bot for future mythical EV gain.
Also, there is no 'colluding' in heads up poker.
An example, say the humans are getting to a river situation with too many bluffs for a given betsize, an exploit for the AI would be to always call. The opposite is also true, if they are bluffing too little it should always fold. The players notice that the AI has adjusted, and adjust their frequencies - now exploiting the AI. By taking an exploitative approach the AI leaves itself open to be exploited, this is not the goal.
If this were rock paper scissors, the AI is doing the equivalent of always throwing each at 1/3 - even when it's opponent throws rock every time. It could switch to paper, but a thinking opponent will now switch to scissors, this will continue until we are back at equilibrium. The AI aims to play poker in this same fashion, having the correct frequencies of actions for a given range in every spot.
Poker isn't about equilibrium, it's about misdirection and exploitation. When the table gets cold, you liven it up by convincing everyone to do a round of straddle.
"Tricking an opponent into thinking it has metaphorically thrown rock" extrapolated into a poker example would be betting larger/smaller, calling more/less, folding more/less than is optimal in a given scenario in the hope that your opponent makes a (bigger) mistake. You're simply hoping he makes more errors than you, the AI instead choses to just make zero mistakes and let the opponents do the rest. You can see this in action for yourself in Heads up limit holdem by playing Cepheus (http://poker-play.srv.ualberta.ca)
I agree that would not happen if two equilibrium-seeking computers played each other. Since the human strategy is unknown, it is possible that equilibrium may not exist or be optimal. Even if it's two computers, if one of the computers has the possibility of choosing a non-equilibrium strategy, then again the optimal strategy may not be to seek equilibrium.
This happens in algorithmic trading, where traders would make a large number of low-valued, bad bets to mislead the algo. Then bet big and go the other direction.