That's consistent with the following modified claim: in the absence of firm knowledge of how agenty a thing "really" is, you will tend to take its unpredictability as an indication of agentiness.

However, I am skeptical about that too; the results of die rolls and coin flips don't seem every agenty to most people (though to some gamblers I believe they do). Perhaps what it takes is a combination of pattern and unpredictability? If your predictions are distinctly better than chance but nothing you can think of makes them perfect, that feels like agency. Especially if the difference between your best predictions and reality isn't a stream of small random-looking errors but has big fat tails with occasional really large errors. Maybe.