I wonder if they were scouring the cast iron every time they cleaned it? Normally a layer of carbon "seasoning" develops and I would think that would limit the direct contact of food and iron.

I clean my cast iron pans with hot water and a brush or scraper only. No detergent (or maybe a small amount to break up oil if I cooked something really greasy). No scouring.

It really doesn't take much exposure to make a difference:

https://www.joghr.org/article/11950-testing-the-efficacy-of-...

https://luckyironlife.com/products/lucky-iron-fish