Earlier quoted context omitted.
I think you're probably expecting to see a 50/50 chance (or higher) of guessing it was a computer. But if you think about it, anything better than 50% would make it more human than a human. So is why the target is lower than 50%
Wait, could you explain how a program that is capable of being mistaken for a human more than 50% of the time is more human than human? I'm pretty sure you're human, with a high degree of confidence, much higher than 50%. So to are you more human than human?
But in this variant, "Do you believe the entity you spoke with to be a human or a program?", a 100% threshold is theoretically achievable. That means you're entirely correct: a 51% vote of confidence is decidedly less human than human. Additionally the 30% threshold they've used is laughably low in this context. Without a control group, even an 100%-confidence outcome probably says more about the beliefs of the judges than the ability of the program to simulate a human.
I don't mean to take away from the no-doubt impressive achievement by the team behind Eugene. I just take issue with the hyperbole in its reporting. But, ya know, the media will be the media, and academics gotta get research grants.