From the article: “To get a TRULY random number sequence, you'd need to rely upon some truly random phenomenon, like the decay of a radioactive isotope.”

How is the true randomness of a physical phenomenon proved? Perhaps naively, I imagine that as an arithmetic-based software RNG is deterministic, so is a physical RNG, as otherwise the underlying physical process would be governed by no physical principals, no? Does random in this case not necessarily mean non-deterministic, but rather no statistically visible correlations?