Marginal utility is a consumption function. By itself, it says nothing about cost or what a producer will do,

> In economics, utility is the satisfaction or benefit derived by consuming a product; thus the marginal utility of a good or service is the change in the utility from an increase in the consumption of that good or service.

https://en.wikipedia.org/wiki/Marginal_utility

By contrast, marginal cost is about change in a producer's cost function with changes in quantity,

> marginal cost is the change in the opportunity cost that arises when the quantity produced is incremented by one unit, that is, it is the cost of producing one more unit of a good.

https://en.wikipedia.org/wiki/Marginal_cost

The commenter starts out using the word "cost" along with the rest of his example which is elaborated upon to describe the MC=MR profit maximizing function.