Predictability is defined in terms of probability distributions, and a price X is typically defined to be drawn from something like Xₜ₊₁ ~ Xₜ + N(μ,σ). The purpose might be to quantify some property of μ or σ or something like that. It means "X₀.ₜ does not hold any information about Xₜ₊₁". The assumption is that any price move reflects new information. It might be a reasonable model for various purposes.
But if this model was "true", RenTec would not work, and the "efficient market hypothesis" is invalidated. Which seems plainly obvious.
Ok, so if the market is not efficient, then it's actually a game (poker-like?) and zero sum. Academically, unthinkable thoughts.
Unpredictability is as much a computational intractability frontier as it is a math problem. We know how to do approximately optimal prediction, but if you have to throw a supercomputer at the calculation and wait until the heat death of the universe to get an answer (which is the essential reality) then it has no value. But if you can grind out small improvements at the prediction frontier on a tractable amount of computing hardware due to algorithm advances and mathematical improvements in more narrow cases, then you have an almost unbounded greenfield to work with and these improvements will generalize well across diverse markets.