The participants all had varying degrees of belief in a just world which were measured with a six-item test. The results of that test correlated significantly with the chance of a favourable self-reported coin toss.
It is meaningless to draw inferences from comparisons to another study, as they were selected differently so there's no control, and it isn't necessary as this study contains a random and varied sample of participants.
Yes, there is a question over the representativeness of Mechanical Turk workers. But even though they're surely different on average from the average human, this study is controlled by comparing them to each other rather than to some pre-determined statistics, so the sampling bias should be largely cancelled out, barring second-order interactions.