When you write "If the others continue to cooperate, their bid is lower and they get nothing" you imply an iterated game. It seems clear from Hamermesh's account that players were only allowed to submit one bid.

Ashley won, but she didn't maximize her win. The smartest thing to do would be to agree to collude, bid higher, and then divide the winnings equally anyway. Everyone gets the same payout, but only Ashley would get the satisfaction of winning. And if someone else bids higher, she's no longer the sole defector, which is socially significant. And, of course, $20 is really not a significant enough sum to play hardball for.