I have read of racers leaving a newly cast block outside for several years to "season it", which means let the metal relax to reach final dimensions before machining. I'd say that was similar to how glass flows over time, since that supposedly explained why window glass in old buildings was thicker at the bottom, but read not-true and just how glass was made in olden days. I think the rust was just an artifact of sitting outside, or at least in a non-conditioned place which gets nightly condensation. Cast-iron is high carbon, which limits rusting to just the surface. It doesn't flake apart like rusting steel, except by saltwater.
Just because racers did it, doesn't mean it was scientifically proven, especially since they keep secrets and copy each other, even if each trick isn't validated. Many tricks start from one-off stories. A foundry which makes cast-iron parts would likely have more history and know the best.