I don't think they're really comparable though. For example: Stainless steel contains 10.5%+ chromium. Carbon steel contains up to 10.5%. Steel (with a non-trace amount of chromium in it) ends up leeching trace amounts of chromium into food. Some cast iron probably has trace amounts of chromium in it, but far less than the 10%+ in steel.

This may also leech chromium into food, but most likely at an even further insignificant amount. It's not part of the study and I am not a chemist, so I don't want to make a conjecture.

Chemistry is complex enough that layman cannot make any assumptions. For all we know stainless steel and carbon steel have higher content than the cast iron, but leach less because it's far more stable.