If I'm interpreting it correctly, that's the probability that this particular PRNG with those particular seeds produces those particular outputs. That's a strawman argument; a PRNG with uniform output should eventually produce any value.

It's perfectly believable that a known broken PRNG from 40+ years ago could be backsolved. The part I doubt is the sensationalized part, that the LLM found the exact method with which it was calculated. It's just a plausible choice for a random oracle. It's a local minimum the LLM settled into.

That's the thing with random oracles, there's likely so many plausible combinations of candidate algorithms and inputs/seed values that the actual truth is almost certainly lost to history.