Everytime I encounter a "daily consumption of ${thing widely believed to be unhealthy} improves health" story it inevitably turns out that the researchers just found another proxy for socio-economic status. Could that be the case here?
I'm thinking something like it's a proxy for self control. Being able to restrict yourself to a relatively small treat shows self control which probably is good indicator for avoiding other foods that might cause heart disease.
I just read the article, and it seems like the ice cream effect was not limited by the amount you consumed. As in, "as little as half a cup a day can reduce your risk of diabetes."