I think the article is confusing. Loss aversion is a property of human decision making, while diminishing marginal utility is a property of some economic system

I think the article is confusing. Loss aversion is a property of human decision making, while diminishing marginal utility is a property of some economic system. The two may be the same, if you believe in (or talk about) subjective utility. But I think subjective utility is a terrible concept to begin with, so better not to go that route.