> Are they really keeping a branch history that's 30k deep? Or is there some kind of hashing going on, and AMD's hash just happens to be more attuned to the PRNG used here?

No, you don't need much branch history to get a vanishingly small probability that any two branches would collide. ~40 bits maybe. The limit will be running out of prediction table capacity I would say. It's possible the better ones are able to cleverly fit competing entries in different TAGE tables whereas the worse ones might start thrashing just one or some of the tables since the test is so regular. It's also possible the better ones just have more prediction resources available (or fewer, bigger tables, or ...).

> Would be interesting to see how robust these results are against the choice of PRNG and seed.

Provided it is somewhat random, I would say very robustly since all those CPUs likely have far more than enough history to uniquely fingerprint every branch even if the PRNG was not a great one.