Each was gambling, not with any vehicle of probability, which had been eliminated from the equation, but with each other's intentions. The Thief indeed took the Seer's bait, stealing the luck needed to affect the flip in defiance of her dare. And in turning to leave, she then posed a dare of her own to the Seer, challenging her to back up the implied threat.
This was the Thief's gamble. She wagered the Seer would not be able to go through with it.