Remember they're doing this so that they can detect it themselves. I'm far from an expert, so maybe I'm misunderstanding something but the way I understand it, they'd be defeating their own tool if they go down this route. If they cycle the logit biases, how can they themselves detect if a random piece of text is generated? Which set of biases do they test?
At the end of the day, you're talking about raw text. There's no option to sign it, or embed metadata or anything like that. You can't even guarantee that you're seeing the complete sample, or even a single sample! If there is a fingerprint, it'll be detectable to anyone, and it'll be easily removed.