analysed data spanning 36 years from over 200,000 adults enrolled in the Nurses’ Health Studies I and II and the Health Professionals Follow-up Study.

So they looked at some data and found that the more heme you ate the higher chance of developing type 2 diabetes. That sounds a lot like a correlation not causation. What was the rest of their diet like? Did the higher heme eaters also consume more in general? I could not find a non paywalled copy of the paper (though I did not look that hard tbh) so cannot tell how good the study was, but from what I read I would not put any stock into these results.

Like so many other dietary studies that make the headlines, they really don't paint the whole, or even a useful part of the picture.