Here's that falling apple thing so that it might be referenced in the full post: https://www.lesswrong.com/posts/5wMcKNAwB6X4mp9og/that-alien-message
I totally remembered the claim wrong. Forgive me for I am after all mere human and not a Bayesian superintelligence:
A Bayesian superintelligence, hooked up to a webcam, would invent General Relativity as a hypothesis—perhaps not the dominant hypothesis, compared to Newtonian mechanics, but still a hypothesis under direct consideration—by the time it had seen the third frame of a falling apple. It might guess it from the first frame, if it saw the statics of a bent blade of grass.
I wonder how many webcam frames were needed for the ancestor simulations; given that we're all actually digital copies in Newcombian-basilisk-hell™ without realizing it.